Note: this only works if you have your own Apache server or have access to the webserver directory (then you can use .htaccess files).
iCal has a nasty input control bug. You can't subscribe to a calendar through an https:// url. The "normal" workaround for this bug is first subscribe to http:// calendar (insecure), then manualy edit ~/Library -> Preferences -> com.apple.iCal.sources.plist and replace http:// with https://. But if you want to share the calendar with "normal" Apple users, you can't ask them to edit their config files... it would be against the evolution (of the species) ;-). The solution:
<Location /dav>
RedirectMatch (.*)$ https://www.secure.tld$1
</Location>
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031102182954997