Strengths & Weaknesses
There are two common warlock PvP builds, and both are extremely strong for different reasons. The major strength of warlocks is from one spell, which you will learn to love (or hate if you are on the receiving end): Death Coil. It is instant cast, does nice damage, heals, AND makes the enemy “Flee in Terror” for 3 seconds, which should give you ample time to cast any spell you wish (most likely fear). A key feature of Death Coil is the “terror” aspect of the description. This is different from the “fear” element of the Fear spell, as it cannot be removed with the Insignia trinkets, nor with the Undead’s Will of the Forsaken. The only way to prevent it is to have high shadow resist, or to use an immunity spell, like Divine Shield or Ice Block.

The key to effective use of Death Coil is to not blow it early. It is on a 2 minute timer, so you should be able to use it once in most fights, but timing is the key. Don’t use it immediately, unless you are low on health. Wait until the middle of the fight. Use Fear first, apply some DoT’s, and use it once you have taken a few hits. That way, you can use all 3 benefits of the spell – the damage, the healing, and the unbreakable fear (which will allow your DoT’s to tick for more damage).
Soul Link Warlocks
Soul Link warlocks are so described because they are specced 31 into the Demonology, tree, using the final skill, Soul Link. This creates a link between the warlock and the pet, transferring 30% of the damage taken by the warlock to the summoned demon. It is essentially 30% damage reduction from every damage source, as long as your pet is alive, and is probably the best damage mitigation spell in the game.
Soul Link warlocks are the survivalists of PvP. While they have no heals, through the use of Healthstones and Drain Life, they can restore life, and can be extremely difficult to kill. They can also put out some good damage, through instant cast DoT’s and Shadowbolts when combined with Fear. Another key talent that most Soul Link warlocks will possess is Master Demonologist, which gives a bonus depending on which pet is out at the time. This is something opponents should look for before engaging a warlock. Most Soul Link warlocks will run around with either a Felhunter (+60 resist all at level 60), or a Voidwalker (10% physical damage reduction). If you are a caster (particularly a mage), you are really going to struggle against a Soul Link warlock with a Felhunter out. Whatever you do, don’t try and kill the pet, as it will have massive resists.

A third trick to note is the use of Fel Domination. This will reduce the cast time of your next summon to .5 seconds, which while not instant, is virtually un-interruptible. This should be used in conjunction with a Voidwalker to give yourself and instant shield. You can also use it to get out a pet that is more suitable for the encounter, say if you have an Imp out when a mage is running towards you.

Conflag (or Fire) Warlocks
These warlocks are at the opposite end of the spectrum from Soul Link warlocks. They will attempt to literally burn you down before you have a chance to react, and apart from mages, have the ability to kill faster than any other class. You can identify a Conflag mage fairly easily, as he will most always be assisted by a succubus pet. If you see a lone warlock without a pet, an invisible succubus is probably nearby, casting Seduce on you as you engage the warlock.
Frontload damage is the key. Trinkets such as Tome of Ephemeral Power and Zandalian Hero’s Charm are excellent, if not essential. Seduce “juggling”, or casting seduce immediately after a spell hits, to prevent the target from doing anything, is a tactic often employed. This means that DoT’s are out the window from the start, as this will break seduce. Only start throwing DoT’s when your opponent will become immune to seduce (on the 4th cast).
Soul Fire, because of its massive damage, is usually the first spell cast. Once that has landed, re-Seduce, and cast a Shadowbolt. If you can Seduce again, cast Immolate. Once they are immune to seduce, use your Death Coil to further prevent them from attacking you. Once your immolate has ticked a few times, use Conflagrate and Shadowburn. This combination (assuming you can pull it off perfectly, without anyone breaking any of your seduces, and you have good quality gear), should kill almost anyone without the need for any DoT’s. It is easy to learn and difficult to master, and heavily reliant on the dice rolling in your favour in a lot of areas. Nevertheless, it is popular and extremely effective.
General Strategies
The most important thing to know when playing a warlock is your enemy. Warlocks are by far the most versatile class when it comes to PvP, and they have a counter to just about every other class. Just for example, they can Silence casters, detect Stealth, and have two forms of crowd control for everyone else.

Be prepared for any eventuation. This means always having a healthstone in your inventory, and having plenty of Soul Shards on hand for resummoning/Soul Fire/Shadowburn. Warlocks also have a multitude of abilities that are rarely used but can be extremely powerful in certain situations. For example, use Detect Invisibility if there are other warlocks around, and you will be able to see their Invisible Succubi. Don’t forget to Banish their pets, and Shadow Ward is great to use against both priests and warlocks. Often this spell is forgotten due to its minimal use in other areas of the game. Finally, always keep Demon Armour active. It adds a small amount of durability, and the shadow resistance comes in handy. The health regen is mostly fluff.
Bandages are also a vital part of being a warlock, because the class relies on converting health into mana. If you have no mana, you won’t be able to kill anything, so you will be Life Taping often. A Fear that lasts for more than a few seconds can give you ample time to repair some of the damage an opponent has done, or allow you to Life Tap back some mana to cast some offensive spells. If you are Fearing to bandage, don’t cast any dot’s until you have healed enough, because you run the risk of breaking the fear early through damage.

Warlocks are the ultimate solo PvP class, and can be very difficult to defeat 1 on 1. Shadow Resistance helps a lot against a Warlock, as does any ability that can break/make you immune to fear. Additionally, a lot of warlock damage comes from dot’s, so there is the possibility that you can bring them down before they can completely kill you, although the dots will often finish you off. Additionally, if you have the ability to remove magic/curses then you can severely frustrate a warlock. Because of the nature of Death Coil, if you can catch a warlock with that on cooldown, or resist one, then you have a fighting chance against them.

Macros
Collect Shards
It’s all too easy to forget to collect shards in the excitement of PvP. With this macro, if your target is below 10%, it will cast Drain Soul. Obviously this is safer to use when there is more than one person attacking your target.
/script if (UnitHealth("target")<10) then CastSpellByName("Drain Soul()") end;
Smart Cursing
This macro will cast Curse of Tongues on spell casters, and Curse of Weakness on melee/hunters.
/script if (UnitClass("target")=="Warrior" or UnitClass("target")=="Rogue" or UnitClass("target")=="Paladin" or UnitClass("target")=="Hunter") then CastSpellByName("Curse of Weakness()") else CastSpellByName("Curse of Tongues()") end;
Self Devour
With a Felhunter out, this will cast Devour Magic on yourself without losing your target.
/script TargetUnit("player"); CastPetAction("4"); TargetLastEnemy()
Arena
Warlocks are somewhat weak in arenas when compared with their dominance in other forms of PvP. They have very poor survivability, unless specced Demonology, and if that is the case their damage potential will be greatly hindered. They do have some use in the arena, however.
Death Coil is still one of the best abilities available. You will be able to use it every fight, of course, as the cooldown is reset on entry. A Felhunter is probably the pet of choice here (unless you have a Felguard, in which case that should be used) – it can be set to attack the healer, and when you see a heal going off, make sure you counterspell. The debuff removal also helps, but to a limited extent.

Affliction warlocks have one excellent combination potential – Seed of Corruption and Unstable Affliction. Because dispelling Unstable Affliction damages and silences the dispeller, Cleanse happy paladins and priests will be wary around you. The only problem is that neither of these abilities are instant. Affliction warlocks will often be targeted first, because they have low survivability and are very dangerous with multiple DoTs and nasty secondary effects. Instant Howl of Terror is also nice.

Demonology warlocks will last longer but have less to contribute damage wise. Felguards are actually quite useful for DpS, and the Stun component of intercept can be very helpful. Don’t set it to auto cast – bind it to a key and choose your target wisely. Use it on a healer mid-heal, or to help out one of your team when they are being attacked. Nightfall/Felguard builds can be quite powerful, but really rely on luck to do much damage, as your DoTs will be dispelled and your Shadowbolts take longer and hit with much less force than Destruction warlocks.
Destruction warlocks can contribute excellent burst damage, but they are probably the easiest of all to kill. If you are not targeted early, your team will have a great chance, but you are likely to die quickly. Immolation, Conflagrate and Incinerate are your spells of choice here. Farm shards and Shadowburn liberally, if you want to help contribute to some spike damage.
Always summon a Soulwell – this will really help your team. It is free. Also buff everyone with both Water Breathing and Detect Invisibility – not only will you be able to see mages/succubi, but it means opponents will spend mana and time dispelling useless buffs off your team.