Well I run with a DK and 3 warlocks plus priest healer on my team and one of the best synergies you can have is ebon plaguebringer deep in the unholy tree. It will give you and your warlock 13% more damage for all your magic attacks and free's up Curse of Agony to be cast on your warlock instead of a CoE. If you are really commited to say the frost tree then spec your warlock for felguard so your pet will get the nice melee attack buff from Icy Talons. For just duo'ing running around (non-instance) I would imagine you would be hard pressed to beat a Frost DK and a Felguard Warlock for trash DPS. For instances you would be hard pressed to beat a Unholy DK and a FG/emberstorm combo.

Allesarelle