Apple released a firmware update for the iMac (iMac EFI Firmware 1.4 Updates) aiming to fix a bug experienced by owners of the Radeon HD 4870-based iMac:
iMac EFI Firmware 1.4 Update addresses issues where iMac computers with ATI Radeon HD 4850 graphics intermittently stop responding and addresses wake-from-sleep issues in Boot Camp.
Directl link to the support download webpage: http://support.apple.com/downloads/_iMac_EFI_Firmware_Update_1_4
