If you have a warlock, I'd sub them in, in the place of the mage.

Mages have three general choices as far as specs.

Arcane, you want 4x Arcane Blast + 1x Arcane Missiles as your rotation.
Fire is dependent on procs, so you'll need something like MSBT (See http://www.youtube.com/watch?v=Fvt-cmKWWbY&hd=1) to announce procs.
Frost is essentially Frostbolt spam, and use your pet at every opportunity.

In 3.3, the water elemental becomes persistent, which should improve the spec. In general its lower dps then the first two, but more mana effecient. It does get Replenishment for the team, and mana regeneration is nice in heroics. For an all caster (non-heroic) team, the snare/freeze effect is quite nice, but with a tank you don't need that.