If you have subscribed in iCal on the Mac to a password-protected calendar hosted on a WebDAV server and sync with your iPad using MobileMe, chances are that you’re unable to authenticate to the WebDAV-based calendar on the iPad.
After turning on calendar synchronization on the iPad an authorization dialog might appear. Tapping Cancel in this dialog has the effect that no calendar data is being synchronized, not even for the non-WebDAV calendars. Depending on when the authentication dialog appears in the synchronization procedure, your Calendar application on the iPad may either stay empty or any new information will not sync.
The only available solution is to unsubscribe from the calendars in iCal.
[crarko adds: I haven't tested this one. Presumably this is a bug in iPhoneOS 3.2 that will be addressed in some future release.]
Mac OS X Hints