The new 2011 iMac line will not allow users to change their hard drives. Whether it be an upgrade in size or an ssd upgrade in speed, iMac users will be stuck with the options that Apple has given them. The hard drives are ordinary SATA 7200rpm drives but come with a proprietary 7 pin power cable making it impossible for the average user to replace.
Sure some people may not need more space or faster speeds but the bigger issue is that hard drives will eventually fail. No matter how awesome the quality, hard drives will fail and with no option to replace them, users will have to back up very frequently and also buy a new computer when their hard drives die out.
This is very similar to the Xbox hard drive situation where Microsoft charged ridiculous amounts of money for an upgrade. Apple seems to be playing this game as well now. Proprietary is not always better (especially when there are absolutely no benefits).
Check out the full story here: OWC