One problem you may run into when using iCal is the program constantly popping up with an error stating "Access to account 'account name' is not permitted." This may happen after syncing from another system, or after migrating to a new account or computer, but upon dismissing the error it may pop up again immediately or whenever the calendars are refreshed.
This happens because iCal has become set up with extra CalDAV accounts, either for another calendar server or for one that is already existing. This may have happened because of a problem with the migration from another computer, or from the synchronization of CalDAV accounts between clients, but either way the system ends up with a duplicate of the current calendar server that is missing the log-in credential information. This results in the system trying to contact the server and getting an error when the log-in information does not work.
Even if you supply the log-in information the account may still not work, or it will duplicate your calendars in iCal, so the best way to clear the problem is to remove the duplicate iCal account.
Go to iCal's preferences, click the "Accounts" tab, and check your CalDAV accounts to see if there are any with missing log-in information. If so then remove those accounts, or if it is the only one for that server then supply the log-in information for it. Keep in mind you should not see an account for "MobileMe," because it is synchronized through the system's sync services and not through a CalDAV account. If a server entry is present with "ical.me.com" as the server URL then remove it, even if it has a username and password combination.