I wouldn't mind hacking on this, making p4aCal a CalDAV client, server, or both.—ErikRose

Okay, the ability for a p4aCal to show events that live on a CalDAV server is almost done. Right now it's an add-on product, CalDAVSubscriber, downloadable from Subversion. When it's actually useful, I'll make a release in the plone.org Products space.