This appears to be a known issue with Google Calendars: http://www.google.com/support/calendar/bin/answer.py?answer=99360#
The Google website says to close and reopen iCal and the message should go away but I have the same issue as you and the calendar I just added just diasappears when I relaunch.
This appears to be an issue with the Google calendar server where they will not allow iCal to write new calendars into their database.
1. Post in the Google calendar forum as there may be others who have encountered this issue and can offer solutions, or at the very least show google there may be something wrong with their calendars: http://www.google.com/support/forum/p/Calendar?hl=en&utm_source=HC&utm_medium=le ftnav&utm_campaign=calendar
2. Rather than adding the calendar through iCal, do it through the Google website. Once I added my new calendar to my Gmail calendar online, I was able to activate it in iCal in iCal > Preferences > Accounts > Gmail > Delegation.
Hope that helps.