My work machine died and I did not have the chance to de-authorize it. To top it off, I had 5 machines listed on my account. I finally found the way to de-authorize my machine. The catch is – you can only do it for ALL your machines on your account. Now I have to RE-authorize all my machines. Yuck.
Here is the direct link to their support page at apple on deauthorization.
Basically, if you are about to trash a machine, MAKE SURE you DE-authorize it or you will need to then RE-authorize each of your other machines this other way.
Weirder! This button apparently does not show up in iTunes (10.1.1.4) unless you have exactly 5 machines installed. I did see the button whilst I had one machine installed. This means that if have one machine authorized, and that machine dies, I have no means of de-authorizing it unless I authorize 4 more machines?!?