It had been in beta testing with the users of Google Apps since quite some time now.
It is now available to all the Google users now.
It uses Google Gears technology similar to how Gmail provides the offline access.
The Offline edition of Google Calendar have several limitations at the moment.
Users would require to manually go offline and online. They are also not able to add events, edit the existing events, search your events or change the settings.
It also does not download all the events in case you use it extensively.