i don't have a bandwidth cap. when I moved here I was initially told I would and it would be 300gb, but they never brought it up to me. The first month I did research and found that many people go over the limit without consequence, so I tested it myself. First month was when I had to reformat twice and decided to watch all of Rome and Spartacus, so I decided to get the massive uncompressed blurays. I went over the limit by 200 something gigabytes, nothing happened. since then I have been over the limit all but once without any extra charges, emails or phone calls.
When I hit the 300gb mark I get a message in my browser telling me so, they just don't enforce it. I recall seeing a message on my comcast account panel saying something about the cap not being enforced, or only in certain cities (Nashville for sure). I took a picture of this on my phone is hopes that it would be enough to play stupid to them on the phone if they did decide to charge me for the extra bandwidth, but I have changed phones since. :/
edit: this sounds about right. I remember making fun of my friend who lives in Nashville and also has Comcast. http://bgr.com/2012/09/18/comcast-300-gb-bandwidth-cap-tests/