The 12th November deadline was set a month ago. You get a free in-game penguin pet for going to the trouble of merging accounts. From today, you need a Battle.net username and password to log in to the game, and anyone creating a new WOW account will need to start with a Battle.net account.
Battle.net is Blizzard's new online platform that will act as a common login, account and player profile for all its games in the style of Xbox Live. It will unify communications, achievements and other features across WOW and forthcoming games like StarCraft II and Diablo III.
More info at the Battle.net FAQ.