The Battlefield 3 beta was only just recently announced officially (the announcement happened at E3). However, we’ve all known about the beta for a long time now since it was announced prior to Medal of Honor coming out.
The BF3 beta will begin in September of 2011. The exact date is not known yet nor is how long the beta will last, what maps will be available, or anything else. More info should be coming soon though with the beta date coming up in just under three months now.
The beta will be available on all three platforms including PC, Xbox 360, and Playstation 3.