As you may know GameSpot is the people working with E3 for news and updates, recently (in the past few hours) they have updated there Arma III release date from Q4 2012 to September 30th 2012 On there searching screen, even though the rest of the there information now says Q3 2012, instead of Q4 2012.
Maybe the release date has been leaked through GameSpot? Or its just an error in Gamespot? I will let you decide.
However don't believe any final release date until Bohemia have confirmed it.