After the launch of Google
Calendar, I think that the idea to integrate applications with it
is passed on the mind of lots of developers. Now it's possible... 
Google has released the Google Calendar Data
API, a set of functions to fully interact with your Calendar, for
example by adding and retrieving events based on a date range, request the
calendar feed etc.
However, my first (and quick) observation to this new set of API makes me
thinking that it's not so complete (and too Java-based) and I hope that it will
soon be updated with new features (and examples too).