DavMail Calendar support is now available, tested with Thunderbird/Lightning and Apple iCal. This should also work with other Caldav clients.
Open iCal Preferences, go to the Accounts tab and click the plus button:
On Snow Leopard, choose CalDAV as account type, enter you Exchange username and password and server address without path:http://localhost:1080
On 10.5 choose a name, enter you Exchange username and password. Open the server options section to define the calendar URL (adjust port to your settings):http://localhost:1080/
Note that iCal will build the full principal path automatically.
Click Add, choose Connect Anyway in warning dialog:
Proceed to OSX directory setup to enable attendee completion.