In preparation for the launch of these new features, on November 12, 2009, all current World of Warcraft players will be required to merge their World of Warcraft accounts with a Battle.net account in order to log in to the game. As our way to say 'thanks' for taking a moment to go through the process, existing World of Warcraft accounts that upgrade to Battle.net (including those that have already switched over) will receive a brand-new penguin in-game pet -- check for him in your in-game mailbox later this week. Starting on November 12, all World of Warcraft players will need to log in to the game using a Battle.net username and password, and anyone who wishes to create a new World of Warcraft account will need to start with a Battle.net account.

Non Battle.net Accounts offline on November 12th
Apparently, if you didn't merge your WoW account to a Battle.net account you won't be able to play the game after November 12th. The good news is, you get a free penguin!

Q: What happens if I don’t merge my World of Warcraft account with a Battle.net account?
A: After November 12, 2009, you won’t be able to log in to the game unless you merge your account with a Battle.net account. You will need to log in using your Battle.net account username (your email address) and password.

Q: How can I obtain the penguin pet?
A: Once you’ve successfully merged a World of Warcraft account with a Battle.net account, the penguin pet will be sent via in-game mail to all existing characters on the merged World of Warcraft account, along with all future characters on that World of Warcraft account. Players who merged previously will also receive the pet this way.

Q: Do I need to merge by a certain date in order to receive the penguin?
A: We plan to remove the ability to get the penguin pet at some point in the future, we haven’t determined exactly when just yet.

Update - Just to clarify, this is directly taken from battle.net, the US version (here) doesn't seem to have the updated information and the process might be different on american servers. You might end up getting the old FAQ when checking the EU site.

Moments ago Emberfin posted a new announcement that confirms what we gathered from the recent FAQ change, meaning from the 5th anniversary of WoW, you can only login to WoW with a merged Battle.net account. For most people that means your WoW login name will change to your email address. Read more below:

A few months ago, we unveiled the Battle.net account, our new way for players to log in to World of Warcraft and future Blizzard games, sign up for upcoming beta tests, shop the online Blizzard Store, and more with just one username and password. This was just the first step in the rollout of the brand-new Battle.net; in the future, players will be able to use Battle.net to participate in cross-realm chat in World of Warcraft, create real-life friends lists, communicate across different games, and a whole lot more.