So what's happening on May 1st? Best case scenario, of course, is the official announcement of a third title in the Diablo series. I wouldn't hold your breath on that one, however -- while it has been rumored for a long time, it seems unlikely that Blizzard would break the news on an unofficial site. But then again, maybe it won't be an unofficial site by then -- if diablo3.com was sold to or otherwise obtained by Blizzard (they obviously own the trademark), and is switching over on May 1st, that would definitely be enough reason for "half the web" to link to it, even without an official announcement of the game.
Either way, we'll keep an eye on it. Keep in mind, also, that Blizzard is having the WWI at the end of June (and both Diablo II and its expansion were released on June 29, of 2000 and 2001 respectively. We're not sure what this countdown means yet, but this summer definitely seems like the right time for an official Diablo 3 announcement.
Update: Whois info on Diablo3.com shows that it's currently owned by Roger Barr, who runs I-Mockery, another link on the sidebar. So this could all just be a hoax or publicity stunt. We'll find out on May 1st, I guess.