Will be a closed beta to start with
During the BlizzCon event this past weekend, Blizzard confirmed that Diablo 3 will have a beta for sure before the title is released. The beta will be a “closed beta” to start with and may be opened for a wider open beta at some point.
Blizzard did not give away any additional information on the beta or when the company expected the game to be released. From the whispers we are hearing, the best guess is that the initial beta will be available to friends and family of Blizzard to start with, and then perhaps eventually an invite to a wider open beta later as the title inches closer to actual release.