Yes, they recently required that all accounts switch over to Battle.net (the cutoff date was sometime early november, after a months notice).

The public reasoning about it is so users can use the same email/password combination for WoW as well as the other upcoming Blizzard games like Starcraft 2, Diablo 3, and whatever their new IP next-gen MMO ends up becoming. The private reasons one can only guess, but I would assume at least that they can do stat tracking much easier with the new system.