By the way, I was completely wrong about Youtube5 converting .swf's to .mp4.

Youtube can normally serve mp4's for mobile devices. I'm guessing this is because not that many of them have flash installed (iPhone). Youtube5 takes advantage of this and sets the "app=youtube_mobile" variable in the video url's query string. It ends up returning an h.264 video. That is all to my understanding at least. It's just what I've learned from porting it.