coordinating my appointments with someone else is hard enough.
Some offer solution.
http://icalshare.com/ – public iCalendar files
http://www.eventsherpa.com/ – proprietary calendaring application that is iCal based. They want you to host your calendar on their site. Which is also ok.
I also looked on MozCal’s calendar sharing ability and thought about using KDE’s KOrganzizer. The two didn’t agree on the same iCal lingo so I have to wait for newer versions.