I have thought about it too..... in arenas it might be deadlier than the mage + priest combo..... provided you can prevent the heals and the dispels.

Don't go shadow. You need healing of some kind. Remember, instant cast Howls are subject to immunity and dimin returns too.

My warlocks are only level 21 or so - but testing on the test realms indicates that in small number combat a holy + warlock team is amazing. It lacks the AoE though but PvE should also be much easier as you would actually have up to 4 tanks in addition to everything else.

If I were to make another combo, it would be 1 priest 4 locks. That or 5 shammies.