Google's help page mentions that users can decide which calendars are available offline. "Calendar keeps you on time, even when you're not online. Offline Calendar allows you to access your events through your browser without requiring Internet access. It's perfect for flaky connections or for when you're in between meetings and have no idea where you're supposed to be next. Note that while offline, Calendar will be read-only - it will not be possible to create, edit, or delete events."
It's interesting that the version briefly available in October 2007 allowed you to "view and edit the next 3 months of your Google Calendar when you're not connected to the Internet". Google decided that it's important to synchronize all the events and that editing the events is too difficult to implement reliably.
{ Thanks, Mark Mathson and Tom Rodman. }