It looks like Dustin Browder's promise for a beta by the end of the summer will hold true as Blizzard are preparing to send out the first invitations any time now.
The first wave of invites will be handed to a large part of the players that helped testing patch 1.5.0 as well to some pro-gamers and members of the press. As time goes, more and more people will be invited to the beta.
Blizzard are reminding players who want to take part in the beta to run the opt-in process, which although does not guarantee an entry gives the chance for at receiving an invitation.
Source: Battle.net
