Always something faster, thinner and better coming around the corner… and amazingly for the same price. I would wait if I were you and you can hold off. I can’t imagine it will be too long now… certainly before lion this summer. My guess is that it will be the same sort of update we are seeing in the MacBook Pros… just better processors and new ports… still, worth a wait of a month or so. Touchscreen is inevitable but I doubt the next revision or two will have them if not even longer.
Apple constantly refreshes their computers. You can always play the waiting game with Apple. If you think you need a computer now, go get it. iMacs are extremely powerful right now. You can get one with an i7 CPU , 16GB RAM , 1GB GPU , and SSDs.
They most likely won’t go back to 30” monitors. The current 27” iMac/Cinema Displays are practically the same resolution. The 30” is a 16:10 ratio while the 27” is 16:9, hence there are slightly less pixels vertically. Both are 2560px wide.
What I expect in the next version towards the end of the year: -Thunderbolt. Current iMacs have 1x Firewire port making it hard to attach professional gear to it. -Lower price. Core i5’s and i7’s will be the standard in their lower end models. -8GB of RAM will be standard in their lower end models (vs 4GB).
Beyond that, I think it’s a guessing game; speculation. I expect a redesign of the iMac to come in mid late 2012. I imagine it would be an entirely new design that would be based on their MacBook Air/iPad experiments.