Apple issued a firmware update on Monday for all 27-inch iMacs that is designed to fix display problems, including flickering screens, that have been reported by hundreds of recent buyers.

"The 27-inch iMac Graphics Firmware Update applies to the graphics firmware on ATI Radeon HD 4670 and 4850 graphics cards to address issues that may cause image corruption or the display to flicker," Apple said in the typically-terse description of the hotfix.

Display issues have plagued Apple's 27-inch. iMac desktop computers since the new machines were introduced in October. Users have reported cracked screens, a yellow tint in part of the display, and irritating flickers that come and go.

The problems prompted one Canadian Web developer to collate the complaints on a specially-built Web site, and may have been one of the reasons why Apple slapped a two-week shipping delay on 27-inch. iMacs earlier this month. That wait applies to the £1,349.00 3.06GHz with ATI Radeon HD 4670 graphics and £1,599.00 2.66GHz with ATI Radeon HD 4850 graphics models.

A thread on Apple's support forum dedicated to the flickering screen problem boasts more than 1,500 messages and has a view count of over 244,000, making it the most-read of those on the iMac forum.

Apple didn't promise that the 638K update would solve everyone's display issues. "If your screen remains black after applying the updater or if you continue to experience image corruption or display flickering after successfully completing this update, contact AppleCare or an Apple Authorized Service Provider," the Apple advisory read.

In fact, just hours after the firmware update's release, user reports were mixed. Several reported that the update had not done the trick. "I have only experienced a mild flicker or two, but I can confirm that the firmware update does not seem to have solved the problem," said a user identified as "OSXAlex" on the support forum. "I noticed a flicker within 5 minutes of restarting after the update."

"Applied the update about an hour ago hoping it would fix my flickers," added "Paul_31" earlier Monday. "Sorry to report that it hasn't -- they seem to be less than they were but not cured."


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However, others said that the flickers were now gone. "Just updated now, since then I didn't find any flickering/tearing of screen," countered "Ianod" this afternoon. "[But] it needs more time ... can't say anything at the moment."

The firmware update can be downloaded from Apple's site, or retrieved using Software Update on a 27-in. iMac equipped with one of the two ATI graphics cards.

Even after the update's release, Apple's online store continued to show a two-week shipping delay for all 27-inch iMacs both in the US and UK.