This is covered by the Nvidia recall program.
"In July 2008, NVIDIA publicly acknowledged a higher than normal failure rate for some of their graphics processors due to a packaging defect. At that same time, NVIDIA assured Apple that Mac computers with these graphics processors were not affected. However, after an Apple-led investigation, Apple has determined that some MacBook Pro computers with the NVIDIA GeForce 8600M GT graphics processor may be affected. If the NVIDIA graphics processor in your MacBook Pro has failed, or fails within three years of the original date of purchase, a repair will be done free of charge, even if your MacBook Pro is out of warranty."
also see this question: Failed GPU Apple Program or Other Repair