If Marco the one who spread the fall rumor is right than it will be October if not later...
As August is summer, September as someone mentioned is the Toy events iPhone,iPods and so, so apple will wait at least a month to not interfere with the toy effect that gives minimum of month.
So logic say, either July, with ml, or October o later.
Which raise the question of Tim cook promise to amazing 2012 year, until now there is nothing amazing.
Only iPad 3 and Mac retina.
Also if we add to the equation the tv rumor then there isn't much time left after September toy event.
Soaybe the iMac is connected to the tv in some way.