Spoken by Jay at Gamescon:

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Q u o t e:
Diablo III's beta will hit by September, hopefully before, Blizzard game director Jay Wilson said during a Gamescom press conference today. When's the game coming out? "When it's ready."

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http://kotaku.com/5831780/bdiablo-iii-beta-will-begin-by-september-still-no-final-release-date

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.