We are running into this issue, not sure if its iCal's fault or a Zimbra issue. The issue we are experiencing is when you make a new appointment inside of iCal, when you invite someone to the meeting you can see the Available meeting times. Well then you click on that iCal will just spin and say checking availability.
Tested with iSync connector and CalDav directly. Issue happens on both.
V4 for iCal running on Snow Leopard 10.6.2