Apparently a batch of the 1TB Seagate hard drives that shipped with iMac systems between May and July of 2011 may fail under certain conditions, and as a precaution Apple has started up a replacement program for the affected iMac systems. The Macs will be either the 21.5-inch or 27-inch models, provided they have been configured with the 1TB drives; other drive configurations such as the 2TB or SSD options are not affected.
If you have purchased one of these iMac configurations, Apple will likely contact you regarding the drive replacement, provided you gave a valid e-mail address when you registered your iMac. In addition, you can check your Mac on Apple's site at the iMac 1TB Seagate Hard Drive Replacement Program Web site. Just enter your computer's serial number in the field and click Submit, and Apple will determine if your system needs to be serviced.
To find your computer's serial number, select "About this Mac" in the Apple menu and then click "More Info." In OS X 10.7 Lion you will see the serial number listed in the resulting window, and in OS X 10.6 Snow Leopard the System Profiler utility will open. In there be sure to select the Hardware section (not a subsection of it) and then the serial number will be listed under Hardware Overview.
If your computer's drive is one of the ones that needs to be replaced, you will need to contact Apple Technical Support, an Apple Store, or another authorized service provider to set up an appointment to service your computer. This information can be found on the drive replacement program's Web page.