The feature was noticed by blogger Mark Mathson, and is being gradually rolled out, but - interestingly enough - only for Google Apps users. Google's FAQ for offline Calendar access is already online and can be found here.
Unfortunately, if you access it while Offline, you won't be able to add anything to your Calendar. According to the FAQ:
"With offline access enabled, you can view event details for calendars which you've selected to be available offline. Note that while offline, Calendar will be read-only - it will not be possible to create, edit, or delete events."
Furthermore, if you're planning to use this feature, you should take care to enable offline access to all your calendars, because only your primary calendar will be synced by default.