Galaxy-S (Vibrant), Android 2.1
If I create an event with the stock Calendar app, I can freely change which Google Calendar the event is associated with. Once it sync's with Google Calendar, however, I can no longer change which Calendar the event is associated with. I can do change any other aspect of the event, and even delete the event, but I can't move it to a different calendar. I can change the calendar from the Google Calendar web page, though.