The Diablo III beta is now open to all. 

Blizzard announced yesterday that for a very limited time, anyone with a account could sign up and join the beta.  The beta will go from today until Monday, April 23 at 10 am PDT.  Players can play any of the five classes and will be able to level up until level 13.

Check out the blog for further details.  Diablo III arrives May 15.