My guess is that at least one of MBP, iMac, Mac mini, or Mac Pro will be announced as an "... and one more thing" at the end, with shipping before the end of September.
How the keynote is going to take place:
Coldplay fades, Tim Cook appears on stage - audience applauses.
10 min on new stores, amount of money they gave to devs.
Federighi comes on stage and talks about iOS 7.
Schiller announces iPhone 5S.
Cook teases about new iPhone 5C and announces it.
Bonus:
They talk about 10.9 and new machines designed to run it.
And scene.
Actually recently Apple has been putting the "less important" updates at the beginning of the keynote and the more important items at the end.
So if there are going to be Mac updates at the September 10th event I think they will happen before they talk about iOS 7 and new iPhones.
At WWDC they did Mac stuff first before the big iOS 7 reveal, and at the iPad Mini even they did all of the Mac refreshes before revealing the new iPads.