According to a post by Tseric on the World of Warcraft General Forum Blizzard may decide that certain largely overpopulated worlds need to be split, and Blizzard has created a mandatory world split procedure and is testing it on Hellscream.
The way the process would work is that a user would log on, be prompted to choose one of two worlds, and when split day happens will be moved. Players that don't choose a world to move to would be moved to the same world as their guild master, if they aren't in a guild they'd be moved with the rest of their arena team, and if they aren't in either a guild or an arena team Blizzard will pick. Also, Blizzard may override your choice of a destination if the problem that caused the split isn't being resolved.
They've got some good reasons for doing this, but I can see a lot of very bad things coming out of this. Guilds getting split up, friends getting split up, guild alliances getting torn apart. The reaction in guild chat when someone brought it to our attention was very negative, but I'm torn. I've seen what long queue times do to gameplay and raid attendance. What do you think? Do the good results outweigh the consequences of moving people around?