Google has added online functionality to its Google Calendar service – although it only brings very limited function in its first iteration.
Google has already rolled out an offline function for Gmail this year, and it looks like disconnected functionality is going to be rolled out further.
However, before you get too excited about offline Google Calendar you should be aware that it's essentially read only – meaning you can't add or cue changes to your appointments on the (offline) fly, and update when you connect.
As Google Calendar's offline help page says: "What can I do offline? 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."
Essentially the functionality in terms of how it stores things is similar to Gmail's when offline – with Google's Gears needed to store a cache of your information that can then be accessed by going to the normal web page, even when not connected.
The chances are that the read only access will be shifted to a more functional ideal going forward, because as you may expect, all of this is very much in Google Beta.
The offline functionality is only available to Schools and Businesses on Google Apps for now - but expect a public version soon.