Wow - a whole thread of talking to myself! The good news is I finally figured out what was going wrong (at the expense of losing about 3 hours at work today!). I'll post the answer here to help other Zimbra users.
Check out the
post by Microsoft employee "Christian Miller".
If I publish an Outlook calendar as private (i.e. only invited users can view the calendar), those users must subscribe via Outlook 2007 or 2010. They
cannot subscribe to a Private Published Calendar via Zimbra (or Google for that matter).
If I publish an Outlook calendar as public (anyone can see it), then I can subscribe in Zimbra - which is what I successfully did 30 minutes ago. I was concerned my public calendar was discoverable by others online at Office.com, but according to the above post the public URL is not exposed anywhere. So while technically someone could find my URL and look at my calendar, it is "hard to guess".