You can now go to your Account page in WoW and sign up for the Wrath of the Lich King beta!!  No word yet on when it will start, or how accounts will be chosen for the beta.

European players can access their account page here.

Once you're in, go down to the icons below, and chose the one that says "Wrath of the Lich King Beta Opt-In" and you'll get this message, after submitting a yes response!

You have successfully opted in for a chance to participate in the Wrath of the Lich King Beta. In the event that your account is selected, you will be sent an email with further instructions. Good luck!

Indeed!  Good luck, now go sign up!