These services have some serious barriers to entry. I created one of these, http://wacchen.com, I think it was the first. When you search for instapaper for video, the first 2 results mention my app. I am not actively maintaining my thing anymore because I realized there was going to be a hiccup in any wide adoption; Youtube. Youtube does not, and probably will never allow people to download that content. 80% of what most people bookmark are youtube videos. Sure some sites out there can be scrapped, but at best your business will be in murky territory. I wrote about this on my blog as well. http://alexkessinger.net/2010/11/15/why-instapaper-for-video...

Someone is going to crack this nut, but they will need to figure out the youtube problem.