I would like to think the next update will be a redesign using a lot of design from the MBPR, mainly it's cooling. I had the 09 iMac 27" i7 and that thing got HOT. It basically burned my screen (you could only see when it was white) but if Apple is going to keep making these more powerful, add retina, their cooling engineering has to be better.
I would not be surprised to see some of the same technology used in an iMac redesign with a slimmer case.
I would not be surprised to see some of the same technology used in an iMac redesign with a slimmer case.