Thanks to Dave Thewlis, executive director of Calconnect, for letting me know today that the organization this week demonstrated interoperable free/busy info flowing between different types of calendar software. The press release is here and there's also a PDF-based presentation online describing the demo in detail.
The demo exchanged free/busy info between seven different calendaring systems: Bedework, Google Calendar, Lotus Notes, Microsoft Exchange, Oracle Calendar, TimeBridge and OSAF's Chandler. CalDAV was a big technological component of making it work.
It will be a while before the fruits of this interoperability work boil down to average calendar users like me, but it's good to see this progress being made at the enterprise level.