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:
Choose a name, enter you Exchange username and password. Open the server options section to enter the calendar URL (adjust port to your settings):
http://localhost:1080/
Note that the field is named Server Address on OSX Lion and Account URL on Snow Leopard. 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.