I have just upgraded my iCal application as the old one was no longer supported....
Anyway, what I don't know is how to find my new iCal application.
How did you upgrade the iCal application?
If you upgraded using the MobileMe Web application you will not see a user interface change on your desktop iCal.
Try using MobileMe: Using Mac OS X Leopard v10.5.x with the MobileMe Calendar, and see if that helps you get on track with syncing.
MobileMe: Troubleshooting MobileMe Calendar issues may also be helpful.