As AppleInsider predicted just the other day, PowerLogix did release an iMac CPU upgrade. That's not the interesting part of it, however... PowerLogix somehow got around the fact that the original iMac processor card houses Apple ROMs.
Mike Breeden at Xlr8yourmac.com wrote them with some questions, including this cool response about how they circumvented the proprietary ROMs:
We provide software that backs up the apple bootrom to the hard drive, then the user installs the iForce. The iForce custom bootrom boots up, takes the bootrom code from the iMac hard drive and flashes the iForce card, then restarts the computer. The computer then operates using a backup copy of the Apple bootrom. The original bootrom image is still on the leftover Apple processor.