disappearing iCal data...
So I didn't hold off, but thought that backing up my iCal would give me a safety net. Well, I lost all calendar info, then went to my backup. I get "restore failed". A very informative message, and I still don't have my calendar info back.
Note: I am in the process of hopefully recovering my calendar. For anyone in a similar state - running 10.5.8 with iCal empty but MobileMe calendars okay - the Apple support document is Article HT4330, and the relevant piece is copied below. The nut of it is that you must set up your MobileMe calendar as a CalDAV account to pull the information down from the cloud, as it were.
If you need to sync a computer with Mac OS X Leopard with the new MobileMe Calendar, follow these steps:
Important: Do not follow these steps until you have successfully joined the new MobileMe Calendar. For more information about joining the new MobileMe Calendar, see this article.
Open iCal Preferences.
Click the plus button at the bottom of the page.
For both Description and Username, enter your full MobileMe email address, including @me.com or @mac.com. If you don't include @me.com or @mac.com, the setup will not be successful.
Enter your MobileMe password.
Click the disclosure triangle next to Server Options.
For Account URL, enter the following without quotes: "https://cal.me.com". Be sure to include the full URL, including the "https://". Note that you do not need to include a closing "/" at the end of the url.
Now that your MobileMe account is set up, iCal will begin downloading the data from the new MobileMe Calendar servers.