I am a little puzzled as to why anyone would want to force Vista or XP on a steamy iMac..... but thats my Mac bias speaking... .osx86project has a story of a forum user who is like this close to booting XP on his new iMac

As a heads up I seem to have gotten to make the machine boot off the local Hard Drive finally (though I'm not exactly sure what I did as I was just fuddling at that time. I'll have to look in detail later) but it still reboots and at least in this config I lost the ability to use F8 on that partition too. On a side to that I also discovered that once the bootmanager is loaded it can read NTFS drives even if the EFI doesn't see it as I put vista back on an NTFS partition on the USB drive and it started loading. Also and this may be a bit more significant....I had installed XP on another partition on the drive and so it showed up on the bootloader. It started do...well..SOMETHING as I had hard drive activity when I selected the Legacy OS boot option. Problem is I couldn't see anything on the screen and the PC rebooted itself after which it told me it couldn't find NTLDR anymore.