Tinytube has recently been formatted for the iphone. It uses the quicktime codec and allows you to watch in portrait or landscape. It doesn't have the "slick" factor but what separates it from the youtube app is that you can watch higher bitrate videos even when on edge, you can choose wifi and just allow it to buffer a little before playing and the vids look great.
www.tinytube.net
www.tinytube.net