Should all the recent rumours pan out then we should have a new 2012 iMac refresh released before the end of 2012. The last major redesign of the model was way back in 2009 so we are definitely due a new one with significant upgrades.
The latest rumours from various inside sources is that the 2012 model will be thinner than its predecessor and also be the first Apple device to feature anti-reflective displays.

AppleInsider cites Taiwanese sources saying that cover glass company G-Tech have just recently doubled their production of cover glass to 3 million in order to match demand for the new iPad. The report also then goes on to say that G-Tech are also expected to supple AR (Anti-Reflective) glass front panels for Apple’s all in on desktop range.

This would be the first desktop Apple Mac to have such a thing, and would mean saying bye bye to not being able to use your computer in high levels of light for example right near a window.
These could come as standard or they may go down the same route as with Macbooks and allow customers to add it onto their custom iMac if they wish to when ordering.

It is thought that these new glass panels combined with a sleeker design and of course the new Ivy Bridge processors are what Apple are using to really set this new iMac apart from its competitors.

The most current predictions are that the 2012 iMac will be released at WWDC in June although this is still unconfirmed.