Others have tried to answer this in other threads.
I have stepped it out more so it might help everyone with this problem as I had this problem too until I figured it out.
Go to iCal
Under iCal menu open Preferences
The tabs up the top will show General, Accounts and Advanced
Choose Accounts
You need to add a new account in the left hand column
Click the + sign at bottom left to add a new account
Under Account Type, you may be able to leave it to the default of Automatic, but I selected MobileMe from the drop down menu
Enter your MobileMe user name and password
Select Create and wait for it all to work.
Phew, iCal is working properly now! I hope it helps
This link describes how to reset up your iphone too
http://www.apple.com/mobileme/setup/iphone_ipod.html
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