Apple has now begun emailing customers for whom it has contact information associated with a purchase of one of the affected machines. The email provides essentially the same information as is posted on the replacement program's support page, but helps ensure that iMac owners are aware of the issue.
Dear iMac owner,
Apple has determined that certain 1TB Seagate hard drives used in 21.5-inch and 27-inch iMac systems may fail. These systems were sold between October 2009 and July 2011.
Our records show that you have an iMac with an affected 1TB Seagate hard drive. Apple will replace your hard drive with a new one, free of charge, under the iMac 1TB Seagate Hard Drive Replacement Program.
Owners of affected iMac machines are covered for three years or until April 12, 2013, whichever provides longer coverage.