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You can set this up either directly in iCal when you create a calendar, or the iSync connector will set it up for you. Note with the latter you probably want to be careful not to have iSync do *both* a CalDAV setup and also handle synching itself.
You're correct about iSync but CalDav is supported in open source. Note you must be running leopard (iCal 3). However you might be able to use webdav with publish/subscribe on older versions of iCal with Zimbra. Did you look at the link I posted?