GM is certainly coming. Developers must have the release version before anyone else to test their apps. And the retina MacBookPro is not running bugfree on the latest build. I guess they want their best MacBook to perfectly run ML.
My guess is today with iOS beta3, around the same time when beta2 came out (4hours from now).
All eyes in the Mac News sites today!
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http://i1-news.softpedia-static.com/images/news2/iOS-6-Beta-3-May-Arrive-Today-10-AM-PST-2.png
iOS 6 Beta 3 coming today at 10AM PST (1PM EST) according to this tweet. (Not Official) ...but it may be a safe bet