According to Slashgear, there seems to be a supply shortage of Apple iMacs in the US which has prompted expectations of a component refresh. Over the last few years, supply availability has been a relatively reliable indicator of new generation hardware ready to be unveiled. The upcoming iMacs will most likely sport Ivy Bridge processors, but there is also speculation that Apple may make the switch to NVIDIA GPU's judging from what the new Macbook Pros are carrying.
Since the new generation AMD and NVIDIA chipsets are more than capable of supporting very high resolutions, it is plausible that the new iMacs may be upgraded with Retina Displays. According to SlashGear, however, there are also conflicting reports that point to an early release of iMacs or Mac Pros in 2013.