You can upgrade memory and disks and airport in any Apple machine without voiding your warranty. I suspect this will also be the case for the new Mac Mini, regardless of what some people are being told. They simply suggest the upgrades be done by authorized reps. Too bad most authorized reps are completely clueless tools who may or may not completely f*ck your toy. I know I have to take my PowerBook in for repair specifically because the asshats at TRG Computers (Newmarket, ON Canada) screwed up my enclosure and keyboard. (You may notice a lot of doublle characters in what I type as ony one symptom of this problem.)
Confirmed upgrading memory doesn't void the warranty (unless you break something):
http://docs.info.apple.com/article.html?artnum=300572
They wouldn't (and can't) use a word like 'should' if they had a warranty voiding policy in place. it would be mandatory to have this clearly spelled out.
Bruno