Chat conversation I just had with Apple:
ME: If I open my iMac to change out the hard drive, does it void my AppleCare warranty?
APPLE: The iMac hard drive is not user upgradable
ME: I realize that is Apple's stance on it, but the fact is that they can be replaced. Just wondering if that voids the entire warranty or if just the new drive wouldn't be covered.
APPLE: If you damage the iMac it would not be covered.
This verifies what I always thought, opening does not void warranty. If you damage something while you are in there, that will not be covered. Aftermarket parts will not be covered either. If you read the AppleCare agreement for iMac, it also verifies this.