A problem has been identified in a small number of Seagate 1TB hard drives used in the 21.5-inch and 27-inch iMacs sold between May and July 2011. According to a page on Apple Support, an affected drive "may fail under certain conditions". It's not known exactly what these conditions are.
If you've registered your iMac with Apple, the company should be sending an email to you telling you about this replacement program. If you didn't register or are unsure if your iMac is affected, you can enter the serial number on this webpage to check.
Apple will replace these affected drives free of charge and the replacement program will run until July 22, 2012.