Lost Remote has a press release from CBS reporting favorably on their YouTube experiment. CBS uploaded 300 clips that were seen by almost 30 million viewers (mostly clips from Letterman), but they also saw a spike in TV viewership, especially on Letterman.
This is a brilliant move by CBS and it's great that they're embracing the viral nature of online video instead of blocking it like a lot of other networks. I haven't watched a Letterman episode in a couple years but I've seen 3 or 4 clips listed in their top 15 watched YouTube videos.
It's win-win. People that don't follow CBS shows can catch the stuff they missed (that everyone is talking about). CBS gets a bump in ratings because those very best bits of their shows do a terrific job promoting CBS shows.
Hopefully ABC, FOX, Comedy Central, and other networks wise up to the positive aspects of YouTube like CBS did and embrace the service.