We pretty much knew this going into Gamescom, but a September start for the beta was given during the D3 Q&A session, and Bashiok elaborated on that time window in a forum post.
Bashiok: That’s weird, they originally had the exact quote in their Inferno article and then pulled it out and only quoted “When it’s ready.” in that new mini-blip thing. I think his original quote on Kotaku was “by the end of September” but I didn’t copy it down.
Anyway, to set expectations straight, beta in August is unlikely. But it shouldn’t be too long after.
The rumor is that Blizzard has had the whole beta time frame set for weeks, and it’s going to start in the 2nd week of September. It’s just that Blizzard being Blizzard, they were not/are not going to tell us that date until it’s nearly upon us, and they are sure they can make it. And yes, “soon™”, “pretty soon™”, “shouldn’t be too long™”, and all other expressions of an indefinite period of time are trademarked by Blizzard’s Project Hourglass.