It states that in the future, Blizzard intends to switch over from World of Warcraft accounts to a centralized account system for Battle.net. Which means that you'll create one account and you'll be able to use it for all of Blizzard's games with online capabilities in the future such as Diablo III and Starcraft II.
The hook is that you cannot upgrade your Burning Crusade account to a Wrath of the Lich King account without accepting the fact that you may be required to open a Battle.net account at some point in the future. You can still play WoW, but you can't upgrade your account for Wrath.
This sounds scary, but it really isn't. If you were in the Wrath beta, you may remember that beta testers had to do something like that. It seems we were not only beta testing Wrath of the Lich King but also the implementation of Blizzard/Battle.net accounts. We logged in using our e-mail address, not our account name. And for our efforts, they awarded us with Chilly the Penguin. It was a pretty smooth transition from using one to the other once it was in place.
So when you see this when you're upgrading your account, I wouldn't worry about it. Go ahead and accept it, and enjoy Wrath of the Lich King. For the average WoW player, when the switchover happens it will probably only take five minutes over your time to plug in your information and you'll never even need to think of it again. If you're a diehard Blizzard fan and will be buying Diablo III, Starcraft II, and whatever other games come our way from them, this will actually be more convenient for you. This is also how achievements will be tracked across all Blizzard games like they have mentioned they want to do.
Plus, they'll probably still give out those adorable penguin pets when they go through with it. So cute!