Re-installing 10.6 does indeed solve the issue. To do this, you need your Snow Leopard installation disk. Just insert it and run the installer again. The whole process takes about an hour. After that, you should install
Mac OS X v10.6.2 Update (Combo). Finally you'll want to run whatever updates are recommended by Software Update
except the 10.6.3 update. This may require several iterations of running Software Update and restarting your Mac.
If you've got several Macs to worry about, it's a good idea to try to download the necessary updates from
Apple - Support - Downloads to a repository instead of using Software Update to download them. (A few aren't available for manual download, though.) This will save you time & bandwidth.
Assuming you've got everything downloaded that you need, these updates will take about 30 minutes.
When done, turn off CalDAV sync in the Zimbra Control Panel (if you have it configured there) and launch iCal to double-check that the CalDAV account is indeed deleted. Then reconfigure the CalDAV account either manually or via the Control Panel.
Some events may have been re-synced back to Zimbra after being rendered incorrectly in iCal. Those you'll have to delete and re-create.