Yes, it’s actually happening. Blizzard explains:
We’re pleased to announce the Diablo III open beta weekend, which offers open access to all players with a valid Battle.net account! Beginning this Friday everyone is invited to log in and help us put the game and servers through their paces in this three day stress test as we march toward the game’s release on May 15. You can begin downloading the Diablo III client right now!
Sound the trumpets!