Xlr8yourmac provides a reader report on a recent article in MacFan magazine (Japanese). According to the report, the Intel iMac Core Duo CPU is mounted in the standard Intel socket, making it replaceable. As a test, a 1.83GHz iMac's CPU was swapped out for a 2GHz one:
However, the author is not sure that it will run completely normally with an upgraded CPU. The author purchased a 2GHz T2500 from a PC shop and installed it in the socket of a 17" iMac and it booted OK. There seemed to be no problems and the About this Mac pane reported a 2GHz duo chip.

The cost of such a minor upgrade ($480 for a 2GHz chip) doesn't make sense at this time but does offer more of a upgrade path than previously available on the iMac.