The guy who won the contest released this video, quality is far from good, but it certainly does look like windows XP installing on an iMac. It will be interesting to see the details on how he pulled this off. The only thing I didn't see in the video was a boot loader, maybe I missed it, but I believe to win the contest you had to be able to select the OS on boot, OS X had to still be on the machine. Does look like he left another partition there for OS X.
The web site is back up, with a download of the required files and instructions. Requires making a custom XP CD with a few updated files slipstreamed in, repartioning your mac, blessing an EFI bootloader and proceeding with a "normal" XP install.