The folks over at Other World Computing, best known for providing affordable upgrades for your Mac, delivered some bad news regarding installing a third-party hard drive in the new Sandy Bridge iMacs.
Apparently, Apple altered the SATA power connector in such a way that removing the hard drive from the system -- or even the bay where it resides -- will cause the hard drive fan to spin at maximum speed. Even further, placing any third-party drive in the machine results in the iMac failing the Apple Hardware Test, even if that drive was replaced with the same model that the iMac is shipped with.
Apple seems to be specifically disallowing hard drives that it doesn't approve of, locking new iMac owners into AppleCare in case something happens to the hard drive within the first three years or using an Apple-authorized repair center. We've touted the virtues of AppleCare many times. However, a good many tend keep their iMacs much longer than three years, and a hard drive is guaranteed to eventually fail. But since upgrading the current iMacs involves removing the screen, even though iFixIt says it is fairly easy to access the internal drive, I wouldn't want to go near that anyhow, and neither would your average customer.