Yes I also notice that BBC Iplayer asks you to allow it to use your bandwidth and become a peer to peer supplier - thereby as soon as your download has finished it kicks in and streams the stuff out to other watchers.
I am sure internet providers have terms and conditions for being a peer to peer user, and if you are on a limited data allowance package, well.. expect there to be complaints very soon that they "didn't know" what that tick box meant...!