Apple has released a software update to solve many of the issues with the computer's display.  

Apple says that a firmware update for its 27in iMac displays should resolve issues such as screen flicker.
An unspecified number of users reported problems with the screen of their iMac, including cracked and flickering displays, and screens that appeared to have a yellow tint.
Apple has issued two software updates that say has resolved the issues with the screens. One, update TS3207, updates the display firmware to “address issues that may cause intermittent flickering”. The second, update DL984, updates the graphics firmware on ATI Radeon HD 4670 and 4850 graphics cards to “address issues that may cause image corruption or display flickering”.  
Some websites, including Gizmodo, said that Apple had offered a 15 percent cash refund to customers suffering from these problems, but the technology company has refused to comment on the speculation.
It's three months since Apple unveiled its update iMac desktops, but shipments of the 27in model have been regularly delayed. Apple has attributed the delays to the popularity of the computer, and said it was “working hard” to fulfill orders “as quickly as possible”. “The 27in iMac has been a huge hit with customers, and we are working to increase supply to meet with strong demand.”
Early reports from iMac owners appear to confirm Apple's belief that the updates will resolve any lingering display issues.
Several iMac users, posting on the Apple discussion forums, said they were no longer experiencing problems with the screen on their iMac.
(Source: Daily Telegraph)