Diablo 3 to the lucky chosen who opted in through their Battlenet account. If you've got one, log into your Battlenet account and download the client now. Like, now now.
Didn't get an invite? Well, first step back from the ledge -- Blizzard will be handing out beta keys through some upcoming promotions and giveaways. So exercise a little patience and just watch a bunch of YouTube videos until your time comes.
[Thanks to everyone who sent this in]