Why? Perhaps the syncing is done through a proprietary protocol. There's no reason why it needs to be mounted as a device. The iPhone could sync through sftp for all we know, there's no techical reason why it needs to be mounted as a drive and then "hidden". If you can access it through Terminal then thats no security at all after all.
Well my iPhone ships before July 17th, so I'll look into it when I get it, but I don't think its going to be all that easy I'm afraid.
Interesting, in Windows you can do a netstat -an to get what connections are active and what ports they are active over. I would imagine you could enable ip filtering for all ports (TCP/IP settings) and then try syncing. If it is using a port to communicate over it would fail miserably. I dunno how to do that in OS X, so I am of little help.
Lastly, is it possible to sniff the data being sent down a USB line? Maybe figure out how iTunes is talking to the iPhone by way of just listening in (as it were).