Since I am trying to move myself away from the iSync (in order to give the CalDAV a fair shake, plus I no longer have an installer for the older iSync connector so I am giving up) I am testing this right now...
Alas, even though the CalDAV implementation has come a long way (due to efforts by Zimbra and Apple engineers no doubt) there are still some limitations. I guess leaving iCal open is the best way around that?
Version 6.0.16_GA_2998.NETWORK Apr 4, 2012 MACOSXx86
Xserve 2x Dual Core Xeons 10GB with Xserve RAID