well, the general consensus is that you spec affliction for PvE raiding at 70....until you have the gear and stats (general thought is around 1-1.1k spell dmg, 190+ spell hit, and ~20% unbuffed crit) to go destruction. destruction has no talents to increase spell hit, so it's very important to be hit capped, and the crit has to be high enough so that improved shadow bolt will be put up as much as possible, and will improve dps more than spell dmg.
while people *could* go destruction in 25-mans if not earlier, their DPS might go thru the roof but their overall raid utility is diminished without shadow embrace and more importantly, malediction (increased CoS/CoE). supposedly 40/0/21 is the spec for the aff warlock in bt/hyjal who is geared for destro but is on mal duty.
it's usually suggested that warlocks wait until around two pieces of tier5 (ssc/tk) to switch, but if you want a personal dps boost, you can switch earlier