I note that the wiki page http://trac.calendarserver.org/wiki/iCal says "1. You need to be running the Mac OS X 10.5 Leopard release for a version of iCal supporting CalDAV." The history list tells me that the page is approx. 2 years old. I imagine that the above statement is no longer precise; now it'll be 10.5 Leopard or 10.6 Snow Leopard.