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/[email protected]/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.