YouTube has Launched Mobile WebApp for iPhone, Youtube mobile site (m.youtube.com) gets full support of HTML 5 for all new Mobiles including iPhones, Videos from Youtube working with HTML 5 gets more quality than the one with Flash player!
This Mobile WebApp gets is really fast, gets new user interface, it gets the same features that found in Youtube.com as "Like"  and "UnLike" and More..

Here's the video by Youtube Introduces the new Mobile WebApp: