"This is my big fear. My guild is rolling Alliance and I want to be able to PvP *and* PvE with them and be able to do pretty well. My biggest hangup at the moment with Warlocks is playing against Will of the Forsaken. How much of an issue is this with BGs and WPvP?"
It's no issue at all in BGs. Playing against the Alliance is much worse. Alliance tarts like that other joker don't realize how easy they have it. Yes you can get screwed by a 1vs1 matchup vs Undead, but this doesn't mean squat in BGs. Stop tunnel visioning on 1vs1, because this doesn't mean anything. Warriors/Rogues already stomp the piss out of Warlocks, forsaken or not. Unless your BG has 0 Paladins, which never happens, you have everything going for you as Alliance. You also go into raids and get Blessing of Salvation, something Horde Warlocks struggle without.