I've found some info on remote wiping other devices in accordance with the activesync protocol over here:
You Had Me At EHLO... : Exchange 2003 Service Pack 2 (SP2) Remote Wipe functionality
I'm starting to think that Zimbra will need a zimlet to plug into the admin console in order to manage the iPhones correctly. That's what exchange seems to offer for it's mobile devices, so a similar tool should be possible in the Zimbra world ?
[EDIT]
Looks like Kerio mail server already support this
How to configure iPhone Exchange ActiveSync® | Kerio Support Center