The latest refresh to the iMac (early 2011) doesn’t allow users to manually change their hard drives.
According to Engadet,
You’ve got no option but to go straight to Apple thanks to a new proprietary cable and special firmware. The standard 4-pin SATA connector has been replaced with a 7-pin configuration that keeps you from hooking up just any old hard drive, and without that magical firmware the iMac won’t even boot.
Michael from OWC wrote a lengthy article about how the restriction is a bad idea, and Marco Arment replied with his view of the matter, which of course is the other side of the coin.
The short of it? Those who intend to purchase an iMac and have no intention of changing the hard drive, or can’t see the importance of it, do so with no worries. For those who intend to, be informed.