my guess is it'll be a simultaneous launch with ios 8 so that all the crossover features will work (handoff, continuity, etc.). so end of september?
Another agree. Late September seems to be the consensus, possibly a delay pushing it to October hahI concur. Rumors have the 6 being released in mid-September. They'll be shipping with iOS8 so it makes sense that Yosemite will be released about the same time.
I'd guess October. iOS 7 came out Sep 18, 2013. Mavericks came out almost a month later, October 22.
Mavericks and ios7 didn't have as much crosstalk.
Id expect them to come together... However; thats a lot of work on Apple's part. Id bet one or the other still with major bugs at release. Batt. Efficient, memory efficieny, or something. Like iOS7 was sluggish at launch.