Someone at Blizzard must have flipped the wrong switch today, and the Diablo III beta was temporarily open as a result. The company has shut down the servers while it fixes the problem, and says it will give more information soon.
A Reddit thread noted the beta had gone open, but the servers quickly went down. Community manager Nethaera commented that the servers are down while Blizzard resolves a problem, and went on to clarify that it's an "accessibility issue." The company expects to provide an update at roughly 11 AM PDT. A similar message was given for European users.
This follows on the heels of an announcement yesterday that the beta will close on May 1, to prepare for the May 15 launch of the game. Once the servers go back online, get your Diablo 3 time in while you can.