Google Calendar provide a xml and iCal feed for 3rd party application. There is a private feed and a public feed, you need to enable the public feed for other to access, and you shoud not publish you private feed.
I do a quick and dirty hack, to write a C program to retrieve calander information from google, seem to be easy. I am using the libxml2.
I plan to write a plug-ins for JPilot, so I can integrate my google calander into my Palm, I don't think is difficult, but still take time, so I stop here for now.
Anyone is interested?