Apple has announced that it will be replacing some HD 6970M cards used in 27-inch iMacs. The computers with the affected cards were shipped between May 2011 and October 2012. (Nick has one. Ed)
The problem affects only 27-inch models with 3.1GHz Intel Core i5 or 3.4GHz Core i7 processors. To be eligible for the free replacement, the last four characters of the serial number must contain one of the following groups (for example, xxxxxxxxDHJQ): DHJQ, DHJW, DL8Q, DNGH, DNJ9, DMW8, DPM1, DPM2, DPNV, DNY0, DRVP, DY6F, F610.
Users who have already replaced the faulty GPUs will be eligible for a refund, reports 9to5mac.