You can pre-order the game on Blizzard's online service gaming website Battle.net right now.
The third game in the Diablo series, Diablo III will be readily available both in retail stores and digitally via Blizzard's website for $60. A retail-exclusive Collector's Edition, packaged with the full game, a behind-the-scenes Blu-ray/DVD set, a soundtrack CD, an art book, and full copies of Diablo II and its expansion pack, will also be available for $100.
Until May 1, you can get yourself a free copy of Diablo III by committing to Blizzard massively multiplayer game World of Warcraft for a year. If you sign up for the World of Warcraft Annual Pass, you'll be able to download a full copy of Diablo III from Blizzard as soon as it is available.
Blizzard did not confirm when the Diablo III beta, which has been released last September, will close its doors.