Browsing around Youtube, I noticed in the status bar (of Firefox) while waiting for a video to load: "Transferring data from v6.cache.googlevideo.com". So Google is apparently hosting some/all of Youtube's videos using their Google Video hosting system, I guess? Interesting.

http://www.googlewatchblog.de/2007/07/06/youtube-videos-demnaechst-auf-google-servern/ that post was published over one year ago :)