If you're going to clone your MAC address on your cable modem to get free Internet, you better be discreet and not download like a motherfucker.
We had a case where some idiot was did this and kept changing his MAC address quite frequently--while at the same time saturating our circuits. Thankfully, we knew which up channel he was so we had a truck go out and over a week track down what specific street the twat was on. Eventually we narrowed it down to his house and he ended up settling with us--I heard it was around $6,000 or so in terms of damage to him.
If you really want to steal someone's Internet access, just break your neighbour's WEP or WPA key (if they have one).