Apple has issued notice of a recall of potentially defective 1 TB Seagate hard drives installed in iMacs sold between May and July of 2011. Apple has offered to replace the possibly defective drives free of charge until July 23, 2012.
The drives, installed in 2011-model iMacs in both 21.5 and 27-inch configurations, may fail under "certain conditions," which has led to the recall. If your iMac is one of those covered under the recall, you can take your iMac in to the local Genius Bar or set up an appointment with an Apple Authorized Service Provider.
Apple will not replace your data for you, so to ensure you don't lose anything, make a full backup of your drive using Time Machine or another backup solution before taking your iMac in for service. Apple does recommend replacing the drive as soon as possible, so if you have one of the affected iMacs but haven't been backing up your data, you'd better start now.