In your "home LAN" example, you really do only need one WebDAV server.
Each client (aka copy of Sunbird) then publishes a copy of its
calendar data to that server. When they exchange data, it's through
the server, not directly from client to client.

Looking at the Sunbird docs, it also appears that you can use FTP to
publish and subscribe to calendars. You'd lose iCal compatibility, but
it could be an effective stopgap until someone has a pure Ruby WebDAV
server ready to go.

FTP also lacks the one major weakness I've encountered with mod_dav:
it maintains its own permissions database, and files are all owned by
the apache process UID. That means that, if you're using mod_dav, you
can't offer WebDAV and direct (i.e., shell of FTP) access to the same
files. Very un-UNIX-y, IMHO.

-- 
Lennon
rcoder.net