If you are a member of Slingbox, there is a Slingbox app that is due out in March. Otherwise, there are currently none that I know of. The reason there are none is because the bandwidth costs for everyone to stream the video from the site would be hundreds of thousands of dollars a month if there were 1/10 of the viewers of YouTube. YouTube's bandwidth costs are about one million dollars a month, making the division of Google, barley profitable. Most apps that stream video have been rejected from the App Store because the 3G network is not strong enough to handle all of the bandwidth.
David