In Activision's quarterly earnings call Mike Morhainmed officially confirmed both an external beta for Diablo 3 and when you can expect to get in on it.
"We began internal company-wide testing last week, The game looks great, and we're currently targeting a Q3 launch for external beta testing."
Q3 would be the months between July & September, that is very soon considering that Mike Morhainmed also says their preparing for 2012 launch. Sadly, no official release date was confirmed. Coming out exclusively on the PC.