November, January and February all have big games set to be released during each month. December, on the other hand, doesn't. If Bethesda truly cares about announcing the game when no other big games are coming out, the best time to announce it would be December.
Well, Cataclysm is set for an early December release, and I'm pretty sure several million people across the world are anticipating that game. But December certainly does have
less big game releases than the other upcoming months. If that's a factor they're considering, then I guess SyltStryder is right about a mid-month announcement, possibly later, after the WoWheads have calmed down a bit.
Maybe a Christmas day announcement will happen after all. ^_^