The new Classes introduced in the upcoming Addon Burning Crusade will make it possible for the Bloodelves, the new Horde race, to become Paladin. At the same time the Draenei will be able to become Shamans.
Read all about „Stealing the Light“ and „Velen’s Vision“ here.
Personally, I do not like this step at all. But I think its Blizzard’s way to make sure there will be enough characters of the new race available.