As noted by MacRumors, Apple's extended service program for iMacs with 1TB Seagate internal hard drives was updated late last week to include iMacs manufactured as far back as October 2009. The program, launched in the summer of 2011, originally covered only machines made during a narrow window of that year. Drives can be replaced at the Apple Store, by an Apple Authorized Service Provider or by Apple Technical Support.
Apple has also extended the duration of this program more than once, and now will cover iMac hard drive replacements with affected drives for three years after the original retail purchase date or until April 12, 2013, whichever ends up providing longer coverage for the computer. You can check if your machine might be affected by entering your serial number on the program page. Of course, you'll need to back up your data prior to replacement, and you may need your OS installer discs depending on how you plan to restore your machine.
Seagate's had past problems with drive reliability, including a firmware flaw that resulted in data loss.