Google had rolled out Gears support for Gmail and have now launched offline access for Google Calendar via Gears. Now you can view, modify and add new tasks and appointments to your calendar when offline and sync the changes as soon as you go online.
You will also be able to change settings and select only the calendars you want offline access to and disable sync for other calendars. Gears for Google Calendar is only for Google Apps users as of now but will be added to other users shortly.
If you have any questions regarding offline access for Google Calendar, you can visit the Google Calendar offline access FAQ.