http://www.reddit.com/r/technology/comments/g62wv/i_woke_up_...
If this is the case, user agent spoofing will not work, and it would be quite a bit of hackery to trick iOS' IP stack into spoofing TTL's for packets from tethered devices.
these can/may be manipulated via ip-tables, but then you still have a huge data-volume to account for...
At least you'll be sure telco guys are not inspecting your packets by any means, be it "teethering detection", blocking some kind of traffic, or something more malicious, like profiling your Google searches. (Timing analysis may still apply.)
With GNU/Linux iptables it's just one rule in the "mangle" table with TTL target.
On *BSD with pf "packet normalization" can be done with pf's "scrub" directive with min-ttl option.
Unfortunately, I've never used iOS and/or OS X/Darwin, so I don't know what are possibilities are there. But I believe there should be some.
(Edit: on reflection, if you know enough to know about the forwarding chains, I'm sure you know that TTL is 8 bits. Sorry.)
I was talking about modifying TTL of the forwarded packets on the phone, to make the packets look like they were originated from the phone itself.