Hmm another piece of the puzzle that I had no idea about. This makes it sound like you can get away with self-signed certs after all. I figure I'll note this in case someone else goes down the same path:
IPhone - Zimbra :: Wiki
Note: The iPhone prefers non-selfsigned SSL connections. If you have a self signed certificate, then it would be a good idea to install the iPhone Configuration Utility to deply a self signed certificate on the iPhone:
Apple - Support - iPhone - Enterprise