They could use custom Penryns at 3.2/3.33 GHz like what happened last year.2. Some have questions whether there will be i7 Mobile chips in the new iMac. I firmly believe there will be as there doesn't appear to be any C2D to move up to for a spec bump. Other manufacturers are using them in laptops soon to ship, and if they can, Apple can.
The Penryn CPUs that have the same clock speeds as the ones in the iMac are similar in price to the bottom two Clarksfields, but Apple is using custom CPUs that we don't know the prices of.Not sure about these people quoting retail prices as evidence that the i7 Mobile is too expensive - since when would Apple pay retail for tens if not hundreds of thousands of processors?
I think it will be similar (at least externally) to the move from the original iMac G5 to the iSight iMac G5.3. Re-design. I for one don't think there will be 'much' in the way of a redesign. Yes, we've seen the thinner google ads but a few less mm here and there would seem logical if the (likely) move to LED screens happens. Thinner yes, full re-design, who knows. I know that from looking at the 24" cinema display next to a current iMac there is very little difference (apart from the chin and colour of the back). So slight redesign, maybe, but let's face it, it's not going to be as drastic as the jump from white plastic to aluminium.
So it looks like the update will be in between a normal update and a redesign. Not a minor update.