Today, Apple released an upgrade to the MobileMe Calendar. The upgrade went fine and afterwards I could see all of my data when browsing to the MobileMe website directly. However it had all (apart from Birthdays!) disappeared from the iCal app on my Macs and from my iPhone calendar. This was despite having up-to-date software on all devices.
Investigating further, I found that the new version expects you to have a server account set up in iCal in order to pick up all of the rest of the data. So went to iCal Preferences, selected the Accounts Tab, and added a new account using my MobileMe account details.
All of my calendars then magically reappeared on that computer, and subsequently automatically appeared on other devices, including my phone, once they had sync’ed up. It seems that the calendars are now server-based, but the upgrade doesn’t always auto-create the server reference in the iCal settings.