You could certainly waste a lot of bandwidth that way. There is no way for the roku to know if a tv is turned on -- at least not when connected through analog cables. There may be some capability (or future capability) with HDMI since that is a digital connection -- I'm just not sure if it supports two-way.
I have thought about that with my channel I'm envisioning. I don't think I would allow more than an hour to play without some user intervention, even if it was only a popup "Hey, you there?" as the other poster suggested.