This just in: a client reported that when she cancelled her MobileMe account, which synchronizes and backs up your calendars to the "cloud," all of her calendar files ON HER MAC were also deleted. Apple has been non-responsive.
A TimeMachine backup would let you recover them, but this little screwup on Apple's part could cost you YEARS of your appointment history if you don't have such a backup. It's not known if this affects all versions of iCal, but it certainly affected her Snow Leopard (10.6) version.
It also wouldn't hurt if you went to the File menu in iCal and chose Export, Calendar Archive and saved that in Documents periodically.
Nobody expected this until it happened.