I run paladin, resto druid, 2 X arcane mage and 1X shaman. It's a very good setup, and arcane mages are the easiest to macro of all the classes for boxers, just set up a click macro that does 4 arcane blasts and then arcane missilles. Grats, you're basically optimized.

Agreed with what fur said about the rotating from missiles and such though - it can get annoying when you're strafing a lot (and you will strafe a lot) in some of the lich king encounters.

If I can, let me advocate dropping a single mage for some other class - my bias says shaman. But warlocks are great too. Simply because you'll love the new buffs/synergy/totems/whatever that another class brings to your group. Totems are amazing, and I always end up dropping one of my mages when I invite a friend to join my group for heroics or raids or whatever.