CalDAV is the Open Standards approach to calendar sharing. It is a little bit late to the game, but adoption is accelerating. Support has been announced by vendors such as Apple and Oracle. The Open Source community has a number of projects underway to utilize this new standard.

General Articles on open calendar sharing:
http://software.newsforge.com/software/06/02/09/2140226.shtml?tid=74
http://ietf.webdav.org/caldav/home.html
http://www.educause.edu/LIVE057
http://www.sscnet.ucla.edu/blogs/consortium/archives/000445.htm
http://tinyurl.com/ya8b8z

Open Source Projects:
Cosmo (CalDAV server) - http://cosmo.osafoundation.org/
Chandler (PIM / CalDAV client) - http://chandler.osafoundation.org/
Mozilla calendar clients - http://wiki.mozilla.org/Calendar:Home_Page