Here's some additional background information:
We've been testing with Mozilla Lightning and Sunbird 0.8. We can subscribe to calendars via CalDAV using the URL
http://server/dav/user/Calendar/, and appointments appear properly both on the client and in the Zimbra web interface. We cannot view tasks, and we cannot subscribe to
http://server/dav/user/Tasks/ since that is not detected as a valid CalDAV store. Lighting reports, "The resource at
http://server/dav/user/Tasks/ is a DAV collection but not a CalDAV calendar."
-TomcaT-