As noted in this post on the Google Code Blog, the Google Calendar now features support for the CalDV protocol:
The Calendar team was the first to launch its Google Data API back in 2006, and in that proud tradition, we're excited to offer an additional way for developers to read from, and write to Google Calendar: the CalDAV protocol. (CalDAV -- an extension of WebDAV -- is an evolving, open standard for calendar synchronization.)
- Google Calendar CalDAV support -- this page explains what you need to do to set up CalDAV support with Google Calendar.
- Enable Google Calendar in iCal -- what you need to do to set up a CalDAV support for Google Calendar in iCal.
- List of known issues -- these are being actively worked on, but you should review the list prior to deciding to try integrating Google Calendar and iCal.
[robg adds: I haven't tested this one; the hint is credited to pamon, as he's the one who emailed me the info on Google's CalDAV support. I wrote up the hint based on the links in pamon's email, so the above words (and any errors) are mine, not his.]

