iMac with defective hard drives, Apple starts the recall

Apple has initiated a program to replace some Seagate 1 terabyte hard drives mounted on iMac 21.5-inch and 27-inch latest generation (iMac 27-inch, almost double performance and Thunderbolt are the only novelty in 2011), because they may "not work under certain conditions" not well specified.
On the Apple website it is stated that the replacement involves a small number of hard disks installed on products sold between May and July 2011. You can enter the serial number of your iMac in the search box to start the test and see if your iMac has a defective disk installed.

The site is accessible at the following address: http://www.apple.com/support/imac-harddrive/
Quoting from Apple website:

Replacement procedure
If your iMac has a hard drive that has the problem, choose one of the options below to replace it:
Apple Store: make an appointment with a Genius.
AASP: find references on the Apple site
Apple technical support: Contact us to know the options of local support.
Apple recommends replacing hard drives concerned as soon as possible. Before the service procedure, make a backup of your data. More information about backup options.

Additional Information
After replacing the hard drive, you will need to reinstall the operating system, other applications and data you backed up, which is why you have to be in possession of the original installation disks supplied with the product.
In the event that you believed to have received a service fee of repair or replacement due to this issue, contact Apple for a refund.
This worldwide Apple program does not extend the standard warranty coverage for affected iMac.

The program covers the iMac systems affected by this issue for three years after the first retail sale of the unit or until April 12, 2013, depending on the coverage period for the user. Apple will continue to evaluate the data and assistance and, if necessary, provide further updates to this program.