So, having acquired a new iPhone, the first test was to hook it up to Zimbra.
The process was surprisingly painless, I selected "Exchange" as the server, plugged in my Zimbra credentials, and was off.
The Calendar works beautifully. New appointments appear like magic, and my selection of Accept/Maybe/Decline updates the server perfectly.
My personal Contacts also sync in beautifully and edits appear to move back and forth the way they should.
The Corporate Directory (which I presume should equate to the GAL), unfortunately, remains blank. Hopefully clever people somewhere will figure out either the right software for the iPhone side or the right tweaks to Zimbra to make this work.
Mail works as advertised as well, although when I read from Thunderbird I need to remember to 'Compact' my INBOX occasionally. Otherwise messages that I copied elsewhere or "deleted" still appear in the phone until the next sync occurs (or I re-delete them there). Changes made from ZWC update correctly.
There does not appear to be any 'Notes' support.
So, overall, excellent out-of-the-box Zimbra compatibility. Not having the Directory/GAL search working will need to be resolved before I can authorize a wider corporate deployment, but it's very usable.