Do you have a 27-inch 3.1GHz quad-core Intel Core i5 or 3.4GHz quad-core Intel Core i7 iMac with the AMD Radeon HD 6970M video card? Have you been seeing white or blue vertical lines in the display? Has your screen randomly gone dark? Does the picture seem generally distorted? If so, your video card may be the culprit and Apple has acknowledged that the AMD card used for those models has been problematic.
Luckily, Apple is offering a free replacement of the video card for machines that are afflicted.
First your 27″ iMac needs to be evaluated to verify that the video card has indeed failed (as opposed to LCD panel issues, EMF distortion, or the like). You can setup an appointment at your local Genius Bar, Apple Authorized Service Provider, or contact Apple Technical Support for additional options. If the evaluation finds that the card failed, Apple will replace the card free of charge for up to three years after the initial sale.