In July 2011 Apple announced the "iMac 1TB Seagate Hard Drive Replacement Program" HDDs that equipped 21.5 and 27 inch iMacs sold between May and July last year which could fail.
On 10/12, the company extended the program, including iMacs sold between October 2009 and July 2011. It is now contacting affected iMac owners who provided a valid email address during the product registration process, to inform them about the program. If you * haven't * been contacted but think you have a 1TB Seagate hard drive, just visit the recall and enter your iMac's serial number to see if you're part of the group.
The exchange procedure can be done at Authorized Technical Assistance or through Apple Technical Support. The program covers affected iMacs for three years from the unit's first retail sale or until April 12, 2013 whichever will provide the most extensive coverage for you.