[citation][nom]TwoDigital[/nom]Is this really a case of hacking? I thought it was rather common if you own your own cable modem (as opposed to renting it from the cable company) to have an option to put in a 'custom' MAC address...not that you should or that it's even a good idea, but I didn't think it was something most people couldn't already do if they own the modem (and hence know the login name/password.)[/citation]
You are probably thinking of the Router, which is a device that plugs into the cable modem and provides firewall and connectivity distribution services. These devices are common to interchange, however the cable modem itself needs to have a stable MAC address or else your provider will not be able to properly maintain your connection (for lack of a better term.) Cloning a cable modem would let your modem look just like your neighbors' modem, so your traffic would look like their traffic. And, since the cable company has little control over the final distribution (for most neighborhoods there is the equivalent of a big ol' RF splitter covering a dozen or more homes) there is no easy way to determine exactly where all those cable modems are. Hence, the problem.