Blizzard has posted a Beta FAQ, along with some information about opting in to the upcoming Beta test. Here’s a quote of the intro; click through for the full FAQ.
While running the System Check application doesn’t guarantee you a place in the Diablo III beta, we wish you luck and hope to see you in Sanctuary.
Click through to read the full FAQ, though it’s vague and non-specific enough that you’ll surely want more details. It also doesn’t address on thing we’ve seen a lot of wonder about: Will there be Hardcore in the beta?
Beta testers are chosen according to their system specifications and other factors, including an element of luck. Our goal is to have a good variety of system types to best test compatibility.
Will there be any other ways to get into the beta besides the opt-in?
We’ll very likely be providing beta entry giveaways through various promotions on our sites, as well as through our official fansites. We’ll have information available on these promotions as they approach.
How many players do you plan to invite to the beta test?
The number of players we invite will be based on our testing needs. If during the course of testing we determine we need more players to participate, we’ll invite more.
What game content will be available in the beta?
You’ll be able to try out all five character classes and experience the early stages of Diablo III from the start of the game through the Skeleton King encounter. You’ll be able to interact with new and returning characters in New Tristram and fight the reawakened evils emanating from the cursed Tristram Cathedral.
Will the PvP arenas be available in the beta?
We don’t plan to include the PvP arenas in the beta test.
How long will the beta test last?
We have not determined an exact date for the end of the beta test. We will notify participants when the beta test is nearing completion.
Will the Diablo III Auction House be available in the beta?
We currently plan to test the functionality of the Diablo III gold-based auction house sometime after the initial beta launch.
Will Mac users be able to participate in the beta?
Yes, Mac users will be able to participate in the beta at the same time as Windows users.