They could release a new iMac on Sept. 12, but I doubt it.
There will be a release of all iDevices: iPad, iPod, iPhone. If iMacs come out, it will be a little kick out to the side. More likely to see a staggered release where iMacs and Minis come out in Oct-Nov. Maybe even December. And maybe next year. The current line up of iMacs and Minis are not "too slow" or outdated in any way.
What is MORE curious is if Apple is going to slowly push down their computer line and focus on their iLine of portables. I'd be suspect about Apple doing 2-year updates on Pros-iMacs-Minis from now onward. Partially because the desktop market isn't Apple's bread and butter anymore. Also because processor needs for 99% of users are already met by the current tech, added to the slowing of advancing processor speeds through this decade into the future (so the tech gurus say).
A Power Mac from 1996 was close to junk by 1999. A Power Mac from 2000 was junk by 2005. A MacPro from 2006 is still going strong in 2012 and will be good through 2014 or beyond. The high-end processing hardware is probably going to slowly become more and more long-lived while iDevices get tossed every 2-4 years due to multiple reasons for obsolescence. The money and focus will now on be on iDevices.
I'm also curious if by 2020 Apple will drop all desktop computers in favor of portable or variations of desktops which are portable. The Mini and iMac, for example, will soon (in 5 years) be closer to the iPad in power. Why build desktops that are close to portables? They'll probably mesh everything into the portable form and ditch the stationary models. If they do so, I wonder if they will allow another company to produce MacPro devices to cater to the slim market reliant on heavy processor power.