One of the requirements for upgrading to the new MobileMe Calendar is to have iOS 4.1 for your iPhone. If you have a first gen iPhone like I do, this is rather hard to do. If you would like to keep syncing your old iOS device and would like to use the new MobileMe Calendar you can without much trouble.
First go ahead and upgrade your Me.com calendar. This will break syncing to your old iOS device.
Next on your iOS device go into the Settings app and select 'Mail, Contacts, and Calendars.' To my knowledge you can swap the order of the next two steps. When doing this for myself, I did it in the order I give here.
Select 'Add Account' and then pick 'Other.' Under the Calendars section, select 'Add CalDAV Account.' The Server you need to enter is cal.me.com. The username and password are your Me.com credentials. Then give the account whatever description you feel appropriate.
Once the account is verified by your device, you will now be syncing your data with Me.com again. If you go look a the calendar app, you will see that everything is in duplicate. To correct this you just need to go back into the Settings app and select your Me.com account. In there you can turn off the Calendars syncing. An options panel will pop up asking what you want to do with the data that has already been synced to the device. Just select to remove it from the device and you should be all set. Your calendar syncing should work as it did before the upgrade to the new calendar on Me.com.
[crarko adds: I haven't tested this one.]
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