Also, have a look at the youtube downloader web service that I have recently published. As you are probably aware, YouTube Google GData API allows replication of the entire YouTube functionality from within your application. However, the only piece missing from this API is the ability to download videos - which is understandable.
By using my web service, you can integrate youtube video downloading capability into your application. Here are the specs:
Service Test using POST: http://www.ecubicle.net/youtubedownloader.asmx
Read more about it at my http://weblogs.com.pk/aspnet