Well, if you got the TBC, you'll know that one of the instances in Hellfire Citadel is The Blood Furnace.
The Blood Furnace will give you the same amount of exp for any level you are.
A level 62 enters the Blood Furnace with a full group and is gaining 500-600 exp per kill, and so will a high level like a 68; even though the mobs were green to the level 68. Exp is supposed to go down if a mob is a lower level then you if I am not mistaken.
Our guild has multiple groups going on and gets about 140k exp/hour.. that's like getting a level in 5-6 hours! The group was 2 rogues, 2 druids, and 1 paladin.. yes no tank. The druids were in cat form for maximum dps, and the paladin was healing.
You get 70k exp per run, and if you have the right group, you'll get 100-140k exp per hour!