Can't sync my new Nexus S to Google Calendar
Is it a Google Apps account?  If so, you need to flick some settings in the domain manager: Settings-&gt;Calendar-&gt;Share all information and/or Share all information, and outsiders can change calendars.  
Have you tried going into the calendar settings and checking `Share this calendar with others'?
Wait a while.  Google Calendar settings changes often take a while to propagate, I've found (at least with Google Apps).  Possibly because the server I'm making my changes on is different from the server that my phone is hitting up.