I agree with everything you've said with one exception that I think is a deal breaker.
The price per unit of the mobile i7 is extremely high. I believe that the entry level i7 mobile chip costs almost 2X as much as the current c2d used in the iMac line.
Considering that Apple have said they want to cut prices it boggles the mind that they would be able to use a part that costs double AND cut costs on the next generation.
No one really knows what Apple is paying for the Core 2 Duos that are used in the iMac. The cost for the T9800 (2.93GHz) and T9900 (3.06GHz) Core 2 Duos are actually pretty close to the Core i7 820QM. However, it is reported that Apple may be using the E8335 (2.93GHz) and slightly less power efficient E8435 (3.06GHz) Core 2 Duos in the iMac line and I can find no reference that gives prices for those parts (they aren't even listed by Intel, rumored to be a special part used only by Apple).
The price quoted by Intel for the T9800 and T9900 is $530 and the cost of the E-series Core 2 Duo that is nearest to those two, the E8400, is $163. However, that particular E-series chip has a thermal design point (TDP) of 65W which means that it is unlikely to be used in the current iMacs. This is where the above referenced E8335 and E8435 come in because they have a reported TDP of just 35W and 44W (respectively). So, we have four possibilities and most likely four price points. It goes like this (all Core 2 Duos with TDP and prices):
T9900/T9800 35W $530
E8400 65W $163
E8335 35W $$$$ (unknown)
E8435 44W $$$$ (unknown)
I suspect that the E8335 and E8435 parts are priced more closely to the high end of this range than they are to the 65W E8400.
In any case, yes, the new Core i7 Mobile processors will likely be more expensive than what Apple is using in the current iMac. However, the price difference is most likely not double as you've suggested (can you give a reliable reference?).
Interestingly, the TDP on the T-series Core 2 Duo + memory and i/o controllers is 44.5W while the Core i7 Mobile + platform controller is 48.5W. This means that the system controllers on the Core 2 Duos add 9.5W versus a 3.5W increase on the Core i7 Mobile processor.
In fact, let's look at the thermal design points for T and E-series Core 2 Duos with their memory controller (MCH) and i/o controller (ICH) versus the new Core i7 Mobile with its platform controller (PCH):
Core 2 Duo T9900 + MCH + ICH = 44.5W
Core 2 Duo E8435 + MCH + ICH = 53.5W
Core 2 Duo E8400 + MCH + ICH = 74.5W
Core i7 Mobile + PCH = 48.5W
This means that if Apple is using the Core 2 Duo E8435 then the new Core i7 Mobile processor with platform controller is actually the lower power solution for the high-end iMac with discrete graphics. It also makes the low-cost, but highly unlikely E8400 option look like real power hog (74.5W versus only 48.5W with the Core i7 Mobile).
I think I've just proven that a thin enclosure using the Core i7 Mobile is a definite possibility. In any case, the TDP is only an approximation of what the true power draw and cooling will require. But given the above the Core i7 Mobile actually looks pretty good.
As for costs, the Core i7 720QM with its platform controller is $404 while the higher-end Core i7 820QM with its platform controller is $586. The high-end 3.06GHz T9900 with its controllers is $569 -- a difference of only $17 from the Core i7 820QM (again, with the caveat that Apple may be using the E8435 rather than the T9900).
Here are these system chip-set prices in a table-like format:
Core i7 720QM + PCH = $404
Core 2 Duo T9900/T9800 + MCH + ICH = $569
Core i7 820QM + PCH = $586
This leaves only the current low-end 2.66GHz iMac with its integrated NVIDIA 9400M graphics chip set. Again, we don't know what Apple is paying for their Core 2 Duos and I've never seen a price quote on the 9400M (but it is reported to be a fairly expensive integrated graphics solution -- after all it was and continues to be the best in that category).
However, the 2.66GHz Core 2 Duo T9550 is listed at $316 and Apple may even be using an older version of the E8335 that ran at 2.66GHz and which is reported to have a TDP of 44W (that could make it less expensive than the T9550). In any case, Apple is almost certainly paying over $316 for the low-end iMac chip set (Core 2 Duo + NVIDIA 9400M), but this looks like a price that will be hard to match with the Core i7 Mobile (because of the starting price on the Core i7 720QM and the fact that the Core i7 can't use an integrated graphics solution like the NVIDIA 9400M). This is one reason why I've suggested that Apple might keep the current Core 2 Duo and NVIDIA 9400M at the low-end of the iMac line-up. Thus, you'd have the Core 2 Duo and 9400M on the low-end (perhaps using the existing motherboard from the current iMacs) and the Core i7 Mobile in the mid and high-end with discrete graphics and a totally redesigned motherboard and case.
