They showed it off, and now Blizzard is laying down the law.  Starting November 12th, you will be required to merge your World of Warcraft account with a Battle.net account in order to log in to the game.  The process, having done it myself, can be mildly annoying, but in order to make you feel less cranky about having to do it, they're giving everyone a brand new in-game pet, a little penguin.  If you've already switched over to get Murky, don't freak out, you'll get one too.  Check your mailbox later this week, you should have him there.

As of November 12th, you will no longer be able to log in to the game servers without a Battle.net account, so get to it.  They will be removing the penguin pet at some point, they're not sure when, so be sure to get it done.  Unfortunately, only existing accounts that merge with a Battle.net account can get the pet.

Why are they making everyone go to Battle.net?  For in game usefulness, so you can use it for cross-realm chat.  Otherwise, it's a "social networking" layer they're adding to things for real-life friends lists, talking across Blizzard games, and the stuff they showed off at Blizzcon.

The US FAQ for Battle.net is available here.