Big news here regarding the future balance of the game, with the expansion:

"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, you will finally be able to play classes that were previously unavailable to your 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." -Frost


And before you let loose with your OMGWTFBBQ, consider this quote from Eyonix:

"Something we have always held to as a core design philosophy is developing classes which are distinct from each other. This means developing a class with it's own abilities that clearly separate it from other classes in terms of how the class plays and operates, both for the player and from a design stand-point.

Early on in the inception of this game, it was a hot debate as to whether factions should have a specific class, which they alone have access to. Some wanted all classes to be distinct from each other, but accessible by all. Others thought that more flavor could be generated by keeping a class unique to a faction. Obviously, if you have one side with a unique class, you should probably give the other faction a unique class as well. Thus, Shaman and Paladins became those unique classes.

However, by linking them in a relationship as unique counter-points, options are closed for our main design goal, which is to keep classes distinct. We want factions to be balanced, but don't want to cut and paste abilities from one to the other and homogenize the classes. If we went that road, there would be little difference or need for a distinct class. We want classes to be different in more than just name-only or superficial appearances.

So, in our desire to keep the classes distinct and open up new possibilities for development of each class, shaman and paladins shall now be a playable class for both factions. This decision comes at a time when we have an opportunity to blend this decision into future development. Namely, with the new races in the upcoming expansion. Prior to the new races, the Paladin and Shaman lent themselves easily to their own factions and not that well to the opposite faction (Tauren Paladin? Gnome Shaman?) With the advent of the two additional races, the choice was made more clear in game design and lore.

In terms of game design, one of the options it opens up is for specific classes in dungeon encounters. We already have several encounters that highlight the abilities of a single class or make use of a classes specific abilities. Shaman and Paladins in the previous design could not participate in such encounters. If killing a creature required a Shaman, the Alliance could never beat the encounter and vice versa. This change allows the two classes to bring their own abilities into a situation which may highlight their class as an integral part of the encounter. "


So it seems as if Blizzard is doing this to protect balance in the most obvious sense, and allow for greater flexibility in other domains of the game. It's a sad loss to see some of the uniqueness of the two factions fall by the wayside, but it might really be for the best, overall.