I've already suggested (multiple times) that Apple may keep the current low-end iMac with the Core 2 Duo and NVIDIA 9400M in its existing design and configuration. Thus, they won't necessarily have to produce a redesigned motherboard and case for the low-end iMac.
This would be a bit unusual, but it's a possibility that might even allow then to lower the entry-level price on the iMac. Now I'm sure many are going to scream, "Who would buy last year's design?" Well that could be part of the plan, many would want to upgrade to the newly redesigned models that will use the Core i7 Mobile. It also plays the clock speeds nicely since the Core 2 Duo's higher clock speeds could still be seen as a "consolation" (so to speak) for those that can't afford the new Core i7 Mobile. Besides, it does seem that Apple will be offering a new keyboard and mouse and that will refresh the lineup even if they don't do a redesign on the low-end models.
I'm not saying this will happen, just that it may be a way for Apple to bridge the current situation with the relatively high price of the Core i7 Mobile and the fact that it can't use integrated graphics. If they don't change the appearance that much then most people probably wouldn't even notice the difference between the old and new industrial design.