There doesn't need to be a cap on bandwidth. Personally, I have stopped using Bittorrent except to download Naruto Shippuuden episodes from Dattebayo for the past month.
I requested to see how much bandwidth I had used from Comcast (I was watching a LOT of online video on CBS.com and others this past month)...... 100GB's. I was like "WHAAAAAAT?!"
Now, each month I usually download about 100GB's of stuff using Bittorrent..... so I'm getting PRETTY dang close to that cap adding my usual downloading AND Bittorrent together, so close that I think the cap, if any, should be a little closer to 400GB's, at least.
That said, I really don't want ANY cap at all. The facts are that people are going to start buying complete movie ISO's online in HD and burning them to Blu-Ray discs (I've just ordered a Blu-Ray burner for my parents computer) so..... bandwidth caps aren't going to work.
Comcast and other internet companies just have to increase their bandwidth and speeds.... that is the ONLY real solution in the end. They are trying to do that now, but from what my cousin who works for Comcast has told me.... they are not moving very fast.