It is possible for them to detect this if they are doing Layer 7 inspection. All it would take is parsing the user agent to see that you're not on Mobile Safari. On the iPhone side, it just does a NAT and theoretically passes all information as the public IP of the phone itself.
Honestly, any respectable nerd is going to have either a) a box to SSH to or b) a VPN endpoint... if you encrypt/encapsulate all traffic originating from your tethered machine there's very little chance they'd be able to catch you.
Assuming the IPv4 TTL issue can be worked around (see elsewhere in thread), a phone that does GBs/month of encrypted traffic over SSH would still be a signal that something suspicious is going on.
Honestly, any respectable nerd is going to have either a) a box to SSH to or b) a VPN endpoint... if you encrypt/encapsulate all traffic originating from your tethered machine there's very little chance they'd be able to catch you.