Okay so I'm ignoring some of what EJ says, this will be the top DPS spec in a realistic raid environment.
http://wowtal.com/#k=RB8TVu3.a5o.warlock.-Hnonc
Priority is pretty complex actually.
If you have nobody else covering these buffs, start with CoE > Shadowbolt. Refresh Sbolt as needed.
Otherwise:
Start out with Soulburned Soul Fire.
Soul Fire (over 80% w/o Buff)
Immolate
Bane of Doom (Bane of Havoc for cleave target)
Conflagrate
Empowered Imp Instant Soul Fire/Soulburned Soul Fire
Chaos Bolt
Corruption
Shadowflame (If you can afford to be in melee range but if using Searing Pain it will be unlikely )
Searing Pain (If under 50% and tank can take aggro)
Incinerate (If over 50% or tank can't hold aggro)
If moving, Procced Soul Fire > Conflagrate (if available) > Corruption/LT (LT if needed ) > Shadowfury > Shadowburn.
Pop Infernal for Hero.
In fights with adds, make sure to Soulburn snipe to keep your shards topped up so you can carry on with instant Soulfires.