Blizzard has posted a note on the US World of Warcraft site reminding players that the beta test for the Cataclysm expansion will be starting soon, and informing them how to opt in to be considered for it.
There's nothing on the European WOW site as yet, so plans on this side of the Atlantic may be less far advanced, but the process for opting in is the same.
You'll need a Battle.net account with a World of Warcraft licence attached. Visit the Battle.net beta profile settings page, make sure you have your system specification uploaded using Blizzard's tool, and check the box to say you're interested in beta tests for the Warcraft universe.
Then sit back and join us in waiting for an email from Blizzard. No, we don't know how long it will be either.