Do not upgrade to the new web-based MobileMe calendar system without first backing up iCal on your computer. There are reports of the web-based upgrade deleting all calendars (and one of those reports is mine). Syncing automatically then wiped out the calendars on my iMac and iPhone.
Fortunately, I had my laptop offline at the time. I made a backup of my iCal calendars (File menu > Export… > iCal Archive) and now have an iCal calendar I can use to restore what’s on MobileMe, which will then sync down to the other devices.
I was lucky. Back up NOW.
Karen Anderson
iPhone 4 Tips 