The chosen path seems to me to be CalDAV under Leopard's iCal 3.0 which I had working last year under Zimbra RC2 hosted demo and OS X 10.5.1 in the attached picture. Unfortunately I can not seem to get this working under our deployed 5.0.1 server and 10.5.2, and need to wait on a vendor for further implementation.
As for Thunderbird/Lightning, CalDAV seems to work okay, but there are a lot of issues maintaining a calendar with this app that you can find elsewhere in the forums. I never got free/busy working but that might be solvable in some capacity, if anybody else here has done it.
Entourage may not be a target, as Exchange uses 'alternative techniques' for WebDAV storage I am gambling. See this
request for more info.
Hope this helps at least a little,
Chris @ DPM