Gamespot has the news that Blizzard will be allowing 'crossover' classes
with the new races promised for the Burning Crusade expansion. The Paladin class, up until now an Alliance class, will be allowed for the Horde race of Blood Elves. Likewise, the Alliance Draenei will be able to choose the Horde Shaman class. From the article: "According to Blizzard, Horde paladins and Alliance shamans will have many of the same talents of their traditional counterparts, though they "will also enjoy some unique abilities to themselves, similar to the priest class' racial specialties." Since this new feature will fundamentally change the asymmetry between the game's two factions, it will presumably have a significant impact on the way the game is played, especially in competitive player-versus-player combat."
It's also likely to somewhat balance the preference between the two factions. A pretty race for the Horde, and what is considered (by some) a very powerful class for the Alliance.