The only problem with the idea that Apple will offer dual-core in the low-end iMacs and quad-core in the high end is that there is no currently available CPU/system architecture from Intel that would allow Apple to design one motherboard that would easily support both options. Right now, dual-core would pretty much mean they would have to stick with the current Core 2 Duo. However, quad-core means that they would have to use the new Core i7 Mobile processor which requires a completely different chip set and motherboard. Frankly, I don't see Apple designing a completely new iMac that would depend upon two such different chips and motherboard designs.
However, early in 2010 Intel will be introducing the dual-core Arrandale processor which most likely be used in the next major upgrades to the MacBook Pros. You can consider Arrandale to be the dual-core cousin of the quad-core i7 Mobile. There are differences (obviously) but they share the same CPU architecture and they appear to use the same motherboard socket.
Frankly, if we don't get a quad-core in the next redesign of the iMac then it probably means that Apple is waiting on Arrandale for the next big upgrade to the iMac. Thus, there is some possibility that the changes in the next iMac will be mostly cosmetic and that the "real" upgrades will come early next year with the Arrandale processor and with a high-end option for the quad-core i7 Mobile.
Another possibility, is that they would keep the current Core 2 Duo iMacs for the low end (i.e. no significant changes, same hardware and design as the existing models) and introduce a totally redesigned high end that would use the quad-core i7 Mobile CPU. Then next year the low end would be replaced by the dual-core Arrandale.
Remember also that neither Arrandale nor the i7 Mobile can use NVIDIA's 9400M (integrated-graphics chip set that is used in today's low-end iMacs). Thus, this makes the "best" hardware choices a bit more complex or difficult.