Members of Reddit shared the news, and there's been plenty of confirmation from subsequent posters.
"So if you don't have a Diablo 3 beta game account on your main Battle.net page, you can log into the beta," noted the opening post. "Not entirely sure if this is a bug, or if they've opened up the beta for the last 13 days to stress test it once this spreads.
"If it's not a bug, ENJOY!"
Whether this is accidental or something Blizzard intended we're attempting to find out.
Blizzard last night announced an end date for the long-running Diablo 3 beta: 1st May. The full game is out two weeks later, on 15th May.