Update 1/14/2013:
See http://support.google.com/calendar/bin/answer.py?hl=en&answer=38847.  Google refreshes it's calendars "every few hours".

Google Calendar has known issues with ical feeds not updating or being delayed. You can report this problem with google by visiting the following page.

Report your problem by, clicking here

 Google Product Forums which talk about this issues from 1/18/10 to at the time of this posting 1/11/13

http://productforums.google.com/forum/#!msg/calendar/iXp8fZfgU2E/bcU8KeydUrIJ