Does opening a mac mini void warranty?

That would be incorrect, there are no parts considered customer-installable for the Mini.

If you think I'm wrong I would recommend calling AppleCare. I was a co-writer on their internal document that describes how to speak to this.  They can find it in their version of the kbase.

The issue issue isn't with the installation, but with the damage that can occur during it.  I refer you to your user manual that states "If you open your Mac mini or install items, you risk damaging your equipment, and
 such damage isnt covered by the limited warranty on your Mac mini. "

No need for that tone over something that's not publicly available knowledge, and prior to your statement just now we had no possible way of knowing what you state (or, claim).

The larger issue really, is that the procedure is not at all like opening a typical tower pc,
and the very real likelihood for causing damage is quite high.

Unlike other parts that can/may be shipped to an end-user, there are no official Apple documents for end-users/owners on how to safely & best take apart these units.

The list of necessary cautions is a long one, starting with (anti-) ESD precautions.

Opening any Apple product voids its warranty.

That being said, after reviewing the Apple care documentation carefully and talking to Apple care on the phone, I discovered that it is really up to the Apple certified technician who services your computer next to decide.

If they deem that in your opening of the computer damaged it, they may choose to void the warranty of that part, or the whole machine, It's really up to them.

So, If you can open it and put it back together without causing any damage or marring of the case, the go ahead!

Apple's warranty information is pretty entertaining:

"Adding memory (DRAM, VRAM) or other user-installable upgrade or expansion products to an Apple computer is not considered a modification to that Apple product. Therefore, it is not necessary to obtain Apple's written permission to upgrade or expand an Apple computer. While Apple strongly recommends that you retain the services of an Apple Authorized Service Provider to perform any product upgrades or expansions, you will not void your Apple warranty if you choose to upgrade or expand your computer yourself. However, if in the course of adding an upgrade or expansion product to your computer, you damage your Apple computer (either through the installation of, or incompatibility of the upgrade or expansion product), Apple's warranty will not cover the cost of repair, or future related repairs."

Wow, cool, where did you find that?
Does that apply to all Apple products?

http://docs.info.apple.com/article.html?artnum=13946

And it applies to all apple computers.  Accessories, monitors, and iPhones are not included.
What it boils down to is that Apple doesn't really care what you do with your machine, but don't come to them for help if you mess something up.

Now, is this only for things that Apple deems to be customer serviceable?
Because according to my MBPs  manual, only the RAM is user serviceable and the hard drive needs to be done professionally (Although i did it anyways).