well, i continued to have issues. i needed to add a new calendar in iCal, and noticed that the calendar changes i made were not showing up at all in itunes. I had a similar issue right after upgrading to Leopard, here is what i did tonight that worked:
1. turn off .mac sync
2. back up ical
3. export all calendars as .ics files to desktop
4. create temp calendar
5. delete all calendars from ical (except temp)
6. close/reopen ical
7. reset sync history in isync
8. turn on .mac syncing
9. open itunes, try to sync iphone (continued to get error)
10. unchecked contacts and calendars, click apply
11. force quit itunes
12. resent isync history again
13. open itunes, make sure contacts and calendars are unchecked, sync iphone
14. check contacts, sync iphone
15. check calendars, sync iphone (also, since i was making calendar changes i had to go in itunes and set the computer to replace calendars on the iphone Info>Advanced>Replace Data)
so this seems to be working... for now. i added an event on iphone & sync'd... all good, then deleted that event in ical and changed another event in ical and sync'd... all good.
i hope this isn't too confusing and helps someone. This:
http://discussions.apple.com/message.jspa?messageID=5798729#5798729 is the thread i referenced the first time i had issues.