If you had used MobileMe to sync calendars and stopped using it, you're Mac is likely confused about what calendar info is valid.
Of course, turn off MobileMe syncing on your Mac and iPad.
Next, reset Sync History on your Mac. Here's an Apple Support article that talks about it: docs.info.apple.com/article.html?path=iSync/3.1/en/9247.html
I hope this helps.