In the pre-invite email, Blizzard says that they're gearing up for the Cataclysm beta, and that a Battle.net account is required to participate. Members of the press with a BNet account who received the email can then sign up for the invite pool.
What we don't know: when it's happening. Press registration for the beta is open until April 15th, but Blizzard specifically states that they have no start date ready to announce. We'll keep you updated with more information, including the beta launch date and any new info that comes out of the beta, as soon as it becomes available.
Right now, if you are not a member of the press and get one of these, it is likely a scam. Practice safe browsing.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.