mac mini warranty
First and foremost, the +mac genius+ I talked to flat out told me that opening the mini voids the warranty. There are NO user-serviceable parts in the mac mini. It is not easily opened or easily serviceable, therefore, do not open. The Apple warranty says different about user-serviceable parts, and does not exclude the mini!
Having said that, decisions are still much to the discretion of the mac genius. If they find anything peculiar such as missing screws, unplugged wires, non-apple parts, etc. They may ask you if it has been opened or not.
If you are asked this question, now you have a moral choice: be honest (yes I opened it) and be denied warranty service, or lie (it was serviced by a certified tech, or I dunno what you mean?) and probably get the warranty service you wanted, saving you some major bucks. Nice choice huh?
So bottom line, don't service the mini if you want to be absolutely certain the warranty (or protection plan) is valid.
mac mini, Mac OS X (10.5.5)