Apple has posted two updates for the 27-inch iMac.
The first, iMac SMC Firmware Update 1.0, “fixes Target Display Mode compatibility issues on 27-inch iMac computers.”
The second, iMac EFI FW Update 1.0, “resolves an issue that sometimes caused high processor utilization while playing audio through the headphone output mini-jack (and) resolves an issue that prevented the display backlight from turning on after powering on the iMac.”
Both updates are available via the links above or Software Update.