If you only read one post about faction imbalance in random BG's this year, it should be this one. The always excellent Cynwise of Cynwise's Warcraft Manual (ask for it by name) does his usual thorough job exploring the numbers behind the game. If you were a fan of Nate Silver's election coverage, then you should be reading Cyn's work, as he does much the same kind of thing with the information behind what's going on in World of Warcraft.
This particular post is too dense for me to do it any justice, but just as a summation, Cyn explores the way faction imbalance affects random BG's in several ways - how perceived imbalance affects player choices, how the overall popularity of the factions plays out, and the way in which dividing players into factions in the first place causes instability in the representation. It's really a remarkable and remarkably well presented post, and I urge you to go read it.