Personally I really like 1 priest 4 locks. If you spec the priest shadow the synergy is awesome, and if you spec holy it helps a lot too as locks with healing can do a lot (hellfire, lifetap, etc).

I think when I hit 80 with them I'll probably respec the priest disc for PvP assuming its still awesome like it is now. I had switched from shadow to holy just before 3.0.2 because shadow had horrible mana efficiency with multiboxing, as spirit tap rarely kicked in. Now, with the new mana efficiency talents and the 20% reduced mana cost for shadow word: pain glyph, my shadow priest has AWESOME efficiency (as SW:P is the main spell I cast), so I have respecced shadow and my mana only ever drops if I have to heal someone and then re-enter shadow form.

Personally right now in outland things are awesome - dot 7+ mobs (just need 1x SW:P and 4x corruption to easily take a mob down in 9-10 seconds), use 2 aoe fears, get tons of xp and loot, repeat.