After the CalDAV calendar update to MobileMe today, I can not longer download my regular MobileMe calendars, but my subscriptions download fine (e.g., US Holidays). So, going to Settings/Mail, Calendars, etc. to turn off, then back on your MobileMe calendar WILL NOT refresh your regular calendars on your iPad. You MUST manually create a new calendar account manually to access the new CalDAV server where your MobileMe calendars have been moved.
Here's the direct link to the Apple server with the instructions for setting up access to the new CalDAV calendar server on your iPad:
http://help.apple.com/mobileme/interface/index.html#mmcc733205
This issue will return to normal when iOS 4.2 comes out for the iPad. It works properly on the iPhone 4 with iOS 4.1 today.
NOTE: If you participated in the calendar beta program, you've already set up this account, so you need to do nothing to continue using it.