we are unable to create private appointments via Apple's iCal
having Zimbra running as a caldav server. Since both Zimbra
and Apple iCal are able to handle private appointments, is there
any chance to get this patched in OpenSource edition?

Zimbra: 7.1.2_GA_3268.DEBIAN5_64 DEBIAN5_64 FOSS edition
Apple iCal: 4.0.4 (1395.7) - Mac OS X 10.6.8

In fact this issue has been marked as fixed in NetworkEdition.