1.) First make an iCal backup: Click on each calendar on the left hand side of iCal 1 at a time highlighting it's name and then going to File Export > Export and saving the resulting calendar file to a logical location for safekeeping.
2.) Go to Apple Menu > System Preferences > MobileMe > Sync -- Uncheck calendars so they do not sync. Calendars no longer sync with the new MobileMe calendar.
3.) Go into iCal > Preferences > Accounts and delete the MobileMe account from here.
4.) Remove the following to the trash and restart your computer:
Home > Library > Caches > com.apple.ical and/or just ical
Home > Library > Calendars > Calendar Cache, Cache, Cache 1, 2, 3, etc. (Do not remove Sync Cache or Theme Cache)
Home > Library > Preferences > com.apple.ical (There may be more than one of these. Remove them all.)
NOTE: Removing these files will remove any shared (CalDAV) calendars you may have access to. You will have to re-add those calendars to iCal > Preferences > Accounts.
5.) Once the computer is back up and running go to iCal > Preferences > Accounts, and add the MobileMe account using your MobileMe @me.com email address.
Hope that helps!
Glad to hear you resolved your issue.
This is how the new MobileMe calendar talks to iCal. All MobileMe users were upgraded several months ago (and received automated emails about the change): http://support.apple.com/kb/HT4037