Remember how Blizzard accidentally posted that Diablo III demo up on Battle.net? Well, it's because they were working on putting it up for today. In order to stress test the Diablo III servers in advance of the May 15 launch, Blizzard has opened up the Diablo III demo from now until 12:01 PM PDT (3:01 PM EDT) today until 10 AM PDT (1 PM EDT) Monday.
You can start downloading the client ahead of the start time by going here and logging in with your Battle.net account. The file is 44MB, so with decent internet, it shouldn't take long at all to download. Everyone except players in Korea can try this open beta. Korean players will get the demo sometime next week, along with Hong Kong and Taiwan.
The demo, according to Blizzard, will let you "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." If you're having trouble waiting until Noon PST, feel free to watch the latest trailer, posted above, to pass the time.