You don't say what operating system you're using. The upgrade to the new MobileMe calendar produces different results, and different problems, with different versions of OSX.
Snow Leopard: during the upgrade process your iCal calendars should be moved to MobileMe (this can take some time and you have to wait for it to happen). Once done, your iCal reads the calendars from MobileMe so any changes made in either place will be visible immediately. If this has not happened then this Apple Tech Note provides instructions:
http://support.apple.com/kb/TS3397
Leopard: when the upgrade is completed iCal on Leopard will not be able to see the MobileMe calendars until an additional process is carried out, detailed in this Apple Tech Note:
http://support.apple.com/kb/HT4330
Tiger: iCal on Tiger cannot read the MobileMe calendars, period. The only workaround is to upgrade to Leopard, or Snow Leopard if possible (Intel Mac required).