1.) Backup all your data:
- Backup your iOS device: http://support.apple.com/kb/HT1414
- Backup your computer Addressbook: http://docs.info.apple.com/article.html?path=AddressBook/4.0/en/ad961.html
- Backup your computer iCal: http://support.apple.com/kb/HT2966
2.) Reset the computer's sync services and then restart the computer: http://support.apple.com/kb/TS1627
3.) Plug in the iOS device and attempt a sync.
--- Check the Info tab in iTunes to make sure calendars are still set to sync.
If syncing is still not working:
1.) SafeBoot the computer: http://support.apple.com/kb/HT1455
2.) While in SafeBoot remove the following to the trash:
- Macintosh HD > Library > Caches (All the contents of the folder not the folder itself)
- Home > Library > Application Support > Sync Services (The entire folder)
--- NOTE: To get to "Home > Library" in Lion: Click anywhere on the desktop and then click on the "Go" menu at the top of the computer screen while holding down the "option" key on the keyboard. You will see "Library" in the menu.
3.) Restart the computer and test.
Hope that helps!