In regards to Apple, I have also heard of some smokers warranties getting void due to damage.
In regards to PC's, technically modifying any hardware voids the warrant. That includes replacing the hard drive, the memory, the LCD screen, the processor etc. Now, you could technically change the memory on some computers (that have the bottom access panel) since you would not need to mess with the "warranty" stickers. The problem is if you don't replace it with the same make and model as originally shipped with, they may notice. Oddly enough, when I was doing repairs
out of my shop I very rarely ran into a computer that was still under warranty.
However, I also used to be a contractor for a nation wide warranty company. All of the work I got from them was computers under warranty. I noticed that the warranties were actually from the store though (such as Bestbuy, Walmart, etc). I was never an "authorized" technician with any of the manufactures such as HP, but that didn't seem to matter, I guess because the actual company was? I'm not really sure if it was because I was doing the work in honor of a store warranty, and if it had any effect on the manf. warranty. Sorry, it is vague, just my experience.
On one other note, I also was looking into contracting for another company that worked on primarily Dell warranties. For the job, they wanted me to become a certified Dell technician which ran around $5000 after all was said and done. I never went forward with this deal, but was always wondering if it was possible for an individual to obtain this or if this was just offered to me because this was an official Dell warranty company.
Some people seem to think that if they were authorized/certified by the manufactures, it would get them a lot more business. In my opinion, if you can, it is a very pricey road (you figure if it is $5000 for Dell, HP can't be far behind). Add them all together you are looking at 15-20k. In my opinion a general end-user is not really going to know the difference between you being Dell certified, and the guy down the block who is not. Like I said, I never even ran into warranty issues in the 2 years I had the store, or the 3-4 years I was running out of my house.
Sorry for the lengthy post, just wanted to tell you what I do know, and what I do not know