World of Warcraft: The Burning Crusade Website has details on a new feature to be introduced in the upcoming expansion for Blizzard's MMORPG that will allow faction/class crossovers. They have further details on this, but here's the general summary:

As we draw closer to the release of World of Warcraft: The Burning Crusade, more of the secrets shrouding the draenei and blood elves are being revealed. In the upcoming expansion, players will finally be able to play classes that were previously unavailable to their faction. The blood elves have harnessed the power of the Light for their own needs, and the Horde recognizes the value of these renegade blood elf paladins. Meanwhile, the draenei have been granted the wisdom and strength of the elements, and a number of draenei now follow the path of the shaman. Read on to learn more about the new class combinations available to players of World of Warcraft: The Burning Crusade.