Amid rumors that Apple has halted production of the 27-inch iMac due to display issues, Apple has released a second firmware update. The update, which is for iMac (27-inch, Late 2009) only:
Updates the display firmware on 27-inch iMac systems to address issues that may cause intermittent display flickering.
The update weighs in at 294KB and can be downloaded here or through Software Update.
The first 27-inch iMac update was released on December 21, 2009 that updated "the graphics firmware on ATI Radeon HD 4670 and 4850 graphics cards to address issues that may cause image corruption or display flickering."