Apple has released firmware updates for its 27-inch iMacs. You can obtain the updates via the Software Update component of Mac OS X’s Systems Preferences app.
27-inch iMac EFI FW Update 1.0 (http://support.apple.com/kb/DL1031) is recommended for all quad-core Intel Core i5 and Core i7 processors 27-inch iMacs. This 2.1MB update requires Mac OS X 10.6.3.
It resolves an issue that sometimes caused high processor utilization while playing audio through the headphone output mini-jack. It also resolves an issue that prevented the display backlight from turning on after powering on the iMac.
27-inch iMac SMC Firmware Update 1.0 (http://support.apple.com/kb/DL1030) fixes Target Display Mode compatibility issues on 27-inch iMac computers. It’s a 397KB download and requires Mac OS X 10.6.3 and 27-inch iMac EFI Update 1.0.