I heard this on NPR yesterday -http://www.npr.org/templates/story/story.php?storyId=1040594... . It pointed out that DVR is decreasing the value of a Thursday night ad slot, since if people watch a Thursday night show on Sunday (after the weekend plans have been made), why should ad execs pony up for the prime time ads? Obviously Hulu can play ads during a certain time period, irrespective of shows, so this doesn't hurt them.
Overall, cable and broadcast TV are not sustainable in their current form. My guess is that they'll eventually go to a la carte style subscriptions at the last moment to avoid completely going under, and not a moment sooner.