Yeah i know that not all ISP's provide with an unlimited download bandwidth. So yeah this could present an issue. But as i stated in the XBOX area in the thread that Overload started about PlayOn, you have to remember that these videos are stored on a server for viewing on the internet in a browser or possibly through a desktop app like the one hulu has.
So really these videos and that are not all that large. It is like watching a YouTube video in a sense cause they are not the greatest quality and they are made to be viewed on any system that supports flash. So really your not getting as big a hit as it seems at first.
While operating PlayOn via my XBOX it takes no more than 2 minutes, at most, to load a episode of whatever i am watching. Be it a 22 minutes episode of a show or a 45 minutes episode of a show that is usually an hour on TV with commercials.
So the actual size of the videos is not that large to download. I mean if you are using the service non stop 24/7 cause all you do is watch TV, then yeah it will eat up a lot of bandwidth. But even just using it for a few hours a day i would wager to guess that i havent downloaded more than a few GB of actual data.