I doubt Apple's gonna release an updated iMac soon. They lowered the price of a RAM upgrade by $100 and maybe made some other small price reductions on the 2011 models. Why would they do that and then release new ones? They probably figure any of the available parts upgrades currently available aren't enough of a change and would not be justifiable business wise. The relatively small profit they would make would not be worth it to them.
Walk into an Apple store and see how many people are checking out the iMacs or MacPros. Everybody's fiddling with laptops, iTouches and iPads, and buying bright fuchsia cases for their phones. These customers want to have the silver laptops with the Apple logos like they see on TV and they want to accessorize with Beats headphones and other gimmicky stuff. During yesterday's keynote they were mostly talking about updates to iOS, like asking Siri for sports stats. I read that desktops are only 22% of their sales. It's probably their current strategy to invest in that other 75-78% of products.
I think the only thing coming down the pipeline is a radically different lineup of desktops next year, probably one that's integrated with a merged version of iOS and OS X for all their stuff, including TV. Their acquisitions and patents seem to indicate they still have some interest in business or pro users. Their little MacPro update was probably their way of saying hang in there, we still care about the desktop market. If they refreshed before all the new stuff comes out next year, they would just end up with a huge backstock of stuff that will be outdated. All the competing operating systems and platforms seem to be working on building an "ecosystem" where everything - computers, TVs, phones, tablets, etc. - is merged.