I've been having trouble on my Windows computer with iTunes 6 asking me to reauthorize every other day or so. It returns an "authorization successful" message (saying 4 of 5 machines authorized, which is correct, so it's not taking away another authorization). However, it will not play my purchased music even then. The only solution is to reboot the computer. Closing iTunes does not solve the problem.
It's starting to annoy me, but I've not found a good solution yet. My neighbor's iTunes crashes if left running overnight, even though it's not playing or being used in any way. It's probably related to Rendezvous in his case. I think iTunes 6 is a little bit iffy on the quality.