I've setup Calendar Server 6 and CalDAV Server Coexistent on the same system, which is quite easy to do, just follow the description at our wiki. You need to be sure that autoprovision on CS6.3 is disabled, LDAP attributes correct set and CalDAV user not exist on CS6.3 Backend.
You can setup user on CalDAV Server, they able to access the Calendar with CalDAV Clients, like iCal, iPhone, Thunderbird Lightning, etc.
Convergence (Patch 10) Limitations:
- Convergence not able to access CalDAV backend (Feature planned for next Convergence Update Release - End 2009)
- Freebusy Lookup for CalDAV User didn't work (Issue is already addressed and will be integrated in upcoming Release)
- Event Invitation is based on Mail with \*.ics Attachment (The Event will not automatically stored in CalDAV User Calendar)
MacOS iCal Limitations:
- iCal always would like to save the \*.ics Attachment in the default Calendar. I've not found any option yet to choose the CalDAV Calendar for the save.
- iCal send mail is based on a AppleScript at: /Applications/iCal.app/Contents/Resources/Scripts/Mail.scpt and it will use per default the MacOS Mail.app. If you, like me, working on Thunderbird and would like to setup Thunderbird as Mail Client for iCal this looks to be a challenge. Thunderbird currently (Version 184.108.40.206) not able to used AppleScript. Therefor it will not working by just change "Mail" with "Thunderbird" in the Mail.scpt script. You need to rewrite the script more or less completely. I just setup my Account on MacOS Mail.app specially for this purpose. More on AppleScript and Thunderbird could be found at this Thread.