Applecare definitely won't cover a new version of the operating system. However, last time, there was some sort of guarantee for new computer purchases if the date of purchase was close enough to the release of Jaguar. I forget how much, maybe 2 weeks?
Could you explain this please. Obviously I am missing something. Does installing a new version of OS invalidates your Applecare or is it just that if you install it they will charge you if you call them with a question instead being free? I thought AppleCare is exclusively bound to the machine regardless of the version of OS...