The Mozilla Calendar (like Apple iCal.app) can fetch iCalendar 
files using the HTTP protocol. The OGo ZideStore server is able to generate 
those iCalendar files from the database on the fly.
<br />
Note that Mozilla Calendar provides little "groupware" functionality and 
the support for it is to be consider experimental. It works pretty well
for viewing calendars, less well for changing ones. It's on our todo list
to see how we can extend Mozilla Calendar to interoperate in a better way
with OGo.
<br />
Our 
<a href="../../devs/docs/SxArchitecture.html">architecture diagram</a>
may also explain some things on a high-level view.