At this stage it's hard to predict when the iMac refresh will take place. To me, the ivy bridge processor adds very little in CPU terms, the GPUs in the 2011 iMacs were fine (ok, so the current stock is better but that is only really relevant to gamers and video editors) especially top-end ones, screen excellent so the pressure on Apple to update is less acute as people imagine.
Another aspect to consider, IMO would be that of SSD capacity. If the new MacBook Pro takes off, I'd imagine there may even be capacity constraints on the new SSDs so even less likely that Apple will share them out in any refreshed or redesigned iMacs. They are also probably waiting for real world experience of the new heat sink system before transferring that to a redesigned iMac.