Mages are king of heroic instances at the moment - mostly because of arcane blast. When geared properly, you can pull 10-13k dps without the druid/lock/dk spell damage debuff, by simply spamming arcane blast. You'll go out of mana quickly, but that isnt really an issue since few boss fights last more than 20 seconds.

Of course, heroics arent really difficult at all, so you don't need any specific class to do them - any tank + healer + 3 dps will do, but the arcane tree really favors mages for heroic 5-mans atm. Add to that the fact that you can decently macro raid dps (AB x 3 + AM) with good mana efficiency and very nice CC which can be good while gearing up.

My aspect on this, having tried a lot of combos, is that paladin + priest + 3 x mage will work very well, but you might still want to consider swapping in a shaman for utility (tremor, ToW, cleanse, nature resist).