Not in iCal. It doesn't exist.
There is Apple iCal Server, but it isn't really iCal. It's CalDAV, which is what you really want.
We're VERY much working on this.
http://bugzilla.zimbra.com/show_bug.cgi?id=6052
Just look at all of the activity. No target yet, but I think we're trying for 5.0 (don't quote me on that, because pmweb says it's not
john