You have until Nov 11 - merge your WoW account and get a free penguin pet.

wow211 - Blizzard sets date for Battle.net...

Blizzard has announced that all World of Warcraft accounts will need to merge with its Battle.net service by November 11, 2009.

There's a FAQ explaining everything you need to know over on Battle.net.

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As an extra incentive - if not being allowed to play WoW without an account isn't enough - all players who merge their WoW account with Battle.net will be given a free in-game penguin pet.

"Starting on October 15, 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," explained the Battle.net FAQ.