When my brother-in-law was running small ISP, we routinely checked TTL of packets and applied fingerprinting when in doubt. The question is, does the ISP even bother to check.

(and there was this guy, who tried to use WinXP connection sharing and misconfigured it. As the radio network had some delays, his DHCP was "closer" to nearly half of the nodes, so he became their gateway. He ate his traffic quota in no time ;P )