If you've got a Battle.net account, you get to play during stress test weekend.
Dying for some early access to one of the biggest games of 2012? This is your weekend. Blizzard is having a 'Diablo III' stress test weekend and is inviting anyone who wants to play to download the client and join in the fun.
There are a few restrictions for the new crop of beta testers, but nothing that should cause too much of an issue. You won't get special access to beta test forums, you'll be restricted in the in-game auction house and you won't be able to level past 13. With all the new classes to try out, you'll still have plenty to do.
Of course, keep in mind that this is intended as a stress test. That means you're going to be dealing with queues, server problems, lag, and all the other stuff that comes with a game that too many people are playing at once.