The thread you reference involves iCal 4 (Snow Leopard) and an unknown version of ZCS. Perhaps you have an old version of ZCS?
I use iCal 4 with ZCS 5.0.20 and it's fine except for some glitches. The most prominent that I've noticed are:
Bug 41002 – Compatibility with Google calendar's 'Show As: Free/Available' setting (Actually this has to do with iCal's "Set as free" not synching. Note: fixed in GnR.)
Bug 42120 – Calendar Appointment Disappears when moving from one Calendar to another (Fixed in 6.0.4 and in the upcoming 5.0.22.)
Bug 40541 – ical notifications box not functioning (What remains of the bug is a bit of an edge case, but it can cause confusion.)
Bug 40833 – CalDav with iCal 10.6 Snow Leopard doesn't respect partstat Bug 41365 – iCal can't edit events in shared calendars