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! From Friday, April 20 at 12:01 p.m. PDT (noon), until Monday, April 23 at 10:00 a.m. PDT you’ll be able to log in, team up with friends, and play each of the five heroic classes to level 13 as you fight to save the world from the impending demonic invasion.
Sound the trumpets! (Compare and contrast you experiences this weekend with our own impressions of the game.)