I must say that it makes me very happy to see that there are people besides me working on a Saturday. Apple has just released the iMac EFI Firmware Update 1.4, an update that promises to fix system freezes on iMacs with ATI Radeon HD 4850 graphics chipsets (reported by us two weeks ago), in addition to resolving issues back in the way sleep at Boot Camp.
Once downloaded, the update application (/ Applications / Utilities / iMac EFI Firmware Update.app) must be run in order for the firmware update process to complete. Once applied, the boot ROM version becomes IM91.008D.B08.
IMac EFI Firmware Update 1.4 requires Mac OS X 10.5.6 or higher and weighs 1.7MB.