I doubt it's possible with iTunes. You could sync over the air via CalDAV but that has a bug (the "year 2145 bug") that you can search on for details; Zimbra 6.0 should include a workaround, and maybe Apple will fix it in iPhone 3.1.
Because of the bug, though, at the moment the best way to access a shared calendar from iPhone is using Safari, thus:
http(s)://<servername>/home/<full_username>/<calendarname>.html
E.g.
Code:
https://zimbra.company.com/home/user@company.com/Calendar.html
Bookmark it and you're good to go.