Over on Lifeisaprayer.com, I posted a detailed tutorial/guide on how to replace the hard drive inside a 24" Intel iMac with an aluminum enclosure (the process is similar on other aluminum iMacs). It's a rather intricate process, so in addition to a few illustrations, I posted a video of the process on YouTube (it's embedded over on Lifeisaprayer.com as well!).
Have fun repairing your iMac! (Please be sure to leave comments on the Lifeisaprayer.com post, and not here).