[azim]

It's unlikely AT&T is doing anything fancy at this point, but there's potentially much more to detection than TTL. NAT devices make an attempt to be transparent at layer 4 and try not to interfere with it. Host OS fingerprinting can rely on a combination of options at that layer as well including but not limited to windowing scaling MSS. If AT&T cared to go the distance, it would be very difficult to get around detection without interfering with the TCP/IP stack.

[vetinari]

Use SOCKS proxy for tethering - problem solved. (OK, it is not that transparent for client, but the detection would be much harder).