After purchasing a Mac Mini, I wanted to replace the hard drive from a 5,400 rpm hard drive to a 7,200 hard drive.
Will this replacement void my AppleCare warranty?
Apple's warranty (AppleCare) is specific about what components are user replaceable/serviceable. The hard drive on an older MacBook for instance is user replaceable, AppleCare used to have the option to send you a replacement hard drive along with a disposable ESD wriststrap for you to install it yourself.
The difference though is the hard drive in the MacBook was almost as easy to access as the RAM in the Mac mini. The Mac mini's hard drive is a different story. In order to access it you have to expose the logic board which increases the chance of damage from ESD if proper precautions aren't taken. Because of this the only user replaceable part in the new minis is the RAM, since the logic board is still pretty well protected when only removing the RAM.
The Magnusson Moss act essentially says they can't deny warranty unless they can prove damage. It wouldn't be hard for Apple to say "The upgrade situation you described to us implies that there was a high risk for ESD damage, which has likely caused such and such issue".
The flip side though is that if an Apple Authorized Service Provider (AASP) installs a third party hard drive your AppleCare will remain intact because the AASP has proved to Apple that its technicians follow all safety rules and procedures when working on Macs, essentially the same reason Apple allows them to perform warranty work. Apple would then not be involved in the hard drive's warranty (though most HDDs have 3-5 year warranties of their own) but the remaining components would still be recognized as "in warranty" by Apple. The most you would need to do is provide a receipt for the service you had done at an AASP.
source: I was "that guy" at the Genius Bar for several years who had to turn people away from warranty coverage for unauthorized upgrades & repairs. As soon as your Mac goes out of warranty I definitely encourage a DIY attitude, however I would try to heed the above advice while it's under AppleCare.
The actual upgrade itself will not void your warranty.
However, if at any part during the process, your Mac Mini breaks, this will not be covered by the warranty. If you can upgrade it successfully by yourself and it works perfectly, then you should not have any concern.
"The definitive answer is defined in the warranty. If you damage the computer doing any unauthorized service, then that damage is not covered by the warranty. Apple service techs will assume that if they find a different hard drive than the one that came with the computer, then they will not repair under the warranty even if your service is unrelated to having replaced the drive."
You can avoid this problem though. In case something goes wrong, you should reinstall the old drive before taking the computer in for service, and leave no evidence you opened the computer up. (You don't have to do this if you do not feel comfortable "cheating").
I know this is old but if anyone else falls into this thread searching for this exact question I want to make sure they know their legal rights with upgrades. There is a little known federal law, ONLY APPLIES IN THE USA, called the "Magnuson–Moss Warranty Act" which says specifically that companies CANNOT require you to use only their service techs and ONLY their branded hardware when doing upgrades. This does still apply to ANY PC and has been successful in caselaw with PC's. If the warranty issue is because of the hardware you installed. Then yes it is NOT covered. But they can not just assume that just because you upgraded a part that you caused the damage. They will have to prove it. So if you hear that BS come from an Apple "genius" or service tech quote the federal law and if they still won't stand. Sue them in small claims and you'll get more then enough to buy yourself an new one with some money left over.