Of course, that doesn't confirm that we're actually talking about the next-gen MMO, or that Blizzard is planning on releasing a game called "Cataclysm" at all (StarCraft: Ghost was also trademarked, and we all know what came out of that). It could be another WoW expansion (though you'd think that WoW would be in there somewhere if that was the case), or it could be a completely separate game. Not that we know of one, but Blizzard certainly is working on all kinds of projects that we haven't yet heard about officially.
So. "Cataclysm." Trademark Blizzard Entertainment. Keep an eye out for it at BlizzCon this year.
Update: We noticed the domain wowcataclysm.com expired on June 26th, 2009. The domain was previously held and parked out in Australia. June 26th is the same day the trademark was filed with the US Patent & Trademark Office. The domain is also now held by GoDaddy, who we know handles Blizzard domains. That's just a little too much coincidence for us to stay quiet about. It's entirely possible Blizzard just acquired the domain name.