How to add YouTube video into custom player?

I have a custom video player on my Website with support for .flv/avi/mpg/whavever videos. The content is fed to my player via an XML playlist. What I want to do is include a YouTube video in my playlist, but need the address of the video file -- uming it is an .flv. Can anyone provide some direction or the syntax of the addresses YouTube uses? These sites like keepvid.com all use their own custom scripts to add the requisite extension, so I'm not able find it there.