Late last week Blizzard announced that it was adding 100,000 more players to the Diablo III beta. All invites have already been sent out to North American players so if you previously signed up for the Diablo III Beta and never heard anything, you may want to double check your email. If you never opted in to the Diablo 3 beta or did not receive an invite you still may have a chance to get in through future promotions, contests and giveaways. Here's information from Battle.net FAQ on opting into the beta. Note this does not guarantee Diablo III Beta access, you'll still need to be selected and provided a key from Blizzard.
"To sign up for the Diablo III beta test or future Blizzard Entertainment beta tests, you first need to create a Battle.net account. You can then opt-in to the beta test for Diablo III, as well as beta tests for future Blizzard Entertainment games, through the beta opt-in process. To get started, simply click Beta Profile Settings in Battle.net Account Management. Please note that you must have at least one Blizzard Entertainment game attached to your Battle.net account in order to set up your profile for the Diablo III beta test. Opting in to a beta test through this method does not guarantee that you will be selected."Blizzard recently announced some fairly major changes to both game play and staff for Diablo III, these changes and ongoing work are probably one of the major factors in Blizzard opening up the beta in hopes of finding major issues and gauging fan feedback on some of the changes that were announced. While an official release date had not been announced, most were anticipating a Q1 release date, with some retailers publishing Jan 2012 release dates, the recent changes most likely push the release to sometime later in 2012. [Source: Battle.net]