When Warhammer Online releases later this year do you think there will be a skewed realm population? The GREENSkin blog seems to think so, but history would disagree with them. Whenever a game has been faction-based the "evil" races tend to be popular during the beta period. This fear of imbalance was a issue in Dark Age of Camelot and in World of Warcraft. During both of those instances beta testers expressed worry that the more evil of the realms would be entirely overpopulated post launch. However, the exact opposite happened to the complete befuddlement of beta testers everywhere.
Everyone was left wondering why this had happened but the truth is that there's no simple explanation. It's not simply a matter of players liking the good guys more than the bad ones, as many would guess. In the case of World of Warcraft, many players would also argue that Alliance 1-60 content was simply paced much better than the Horde side. Of course with WoW there was only one class specific to each side (Paladin and Shaman) until a patch much later. In Warhammer Online each side has a bevy of classes all to their own. While all of these classes share similarities mechanically, they do differ in terms of flavor. So you can see how even these two games have different factors affecting realm balance.
That's only scratching the surface as there are lots of worries to have with any MMO beyond even realm balance. There's even the possibility that players coming from WoW to WAR might decide to break tradition and flood the evil races over the good ones. All we know is that we'll be in the thick of it playing our Gobbos, Orcs, Chaos and Dark Elves.