Directly from Battle.net" "Beginning with World of Warcraft®, the new Battle.net account grants access to Blizzard Entertainment games, websites, and online services - all with a single e-mail address and password."

Wondering what this is going to be like, we dig a bit deeper finding:
"Q: What is a Battle.net Account?
A: As part of our ongoing improvements to Battle.net, we are introducing a centralized account system that will let players manage all of their Blizzard Entertainment games, including World of Warcraft and future games, in one place without having to remember multiple sets of login information."
Which tells us that we can manage all our Blizzard games through that. But, is this from a page ingame or a web page or even a strand-alone program like Steam®?

"Q: What are the benefits of creating and using a Battle.net Account?
A: The new Battle.net Account allows you to log in to World of Warcraft, manage purchases at the online Blizzard Store, keep track of CD keys for your Blizzard Entertainment games, access our websites (such as the World of Warcraft Armory), opt in to upcoming beta tests, and more using a single login -- just your email address and a password. In the future, we plan to add more features, including tools to make it easier for friends to communicate between games and to help bring our community of players closer together."
Looks like we are forced to use this or we will not be able to play WoW (And other Blizzard games?)

Full page here.