The Unofficial Apple Weblog is reporting that the Mac mini is indeed upgradable. Not only is the RAM relatively easy to upgrade (there's an image of the Mac mini with case removed to prove it), users can also install an Airport card and a (presumably only Apple ones) bluetooth module. They also state that opening the machine does not void your warranty, unless you break something while opening it. I have previously heard this but believed it to be wishful thinking. The machine will also boot headless, which means it would make a good small server.