Sync errors between iCal and Google Calendars
This is a common error. The iCal auto setup has created an incorrect address for you. There should be no "slash" / at the end of the address i.e. it should be

/calendar/dav/youremail@domain.com/user

To fix it, go to Preferences / Accounts and change the address, removing the final / from the Server Path setting.

AFAIK Lion no longer has this bug.