Over in the Articles section, 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 in the article as well!).
Intel iMac Teardown and Hard Drive Replacement - DIY/Guide
Have fun repairing your iMac! (Please be sure to leave comments on the guide post, and not here).