The obvious conclusion here, of course, is that Blizzard is ramping up for a Diablo 3 announcement -- no doubt sweet, sweet candy to the ears of Blizzard fans everywhere. And that's not a huge assumption -- it's been rumored for a while, and what other reason would Blizzard have to start marking their territory? Of course, as the owners of the Diablo trademarks, Blizzard could probably have claimed the domain any time, though from the post on the new site,
At any rate, if you're going to be optimistic about there being a Diablo 3, the new date to mark on your calendars is Saturday, June 28th -- that's the date that the Blizzard Worldwide Invitational starts in Paris. Last year, they used the event to announce Starcraft 2, so who knows -- maybe this year we'll see Deckard Cain in a cinematic saying, "It's about time."
Update: Actually, the I-Mockery guys said they gave it to Blizzard for free, just because they were Blizzard fans, which is kind of cool. Also, the redirect will apparently last until June 1st, at which point, Blizzard will take over Diablo3.com. Our guess is that then it'll redirect to Blizzard.com (see diablo.com), at least until an announcement is made.