It is possible to steal Cable Internet if you know what your doing (which I don't). I also know that cable companies will just shut off the service at the source if they find out, but won't do anything about it (legally). It's not worth their time and is too difficult to prove wrongdoing on you part.

One of my former roomates is a compsci major, and he did this for us during college. I do remember him saying it had something to do with a filter they put on your line. We had free cable TV and free internet for about 3 months. Then one day, we came home, tv and cable didn't work, and there was a couple of Comcast (the local provider) brochures stuck in our door, with a handwritten note on one that said "CALL US TO GET YOUR SERVICE BACK."

We called like a week later and they came out and did the official installation and we paid for cable for the next 2 years.