http://s.ytimg.com/yt/img/master-vfl33450.gif
Found in YouTube's CSS file.

"My best guess is that YouTube’s partnering up with musicians/music studios to put those buttons on the YouTube pages that are featuring the musicians/music studios’ music video, and link those buttons to iTunes Store and/or Amazon MP3 so the viewers can buy the songs." http://googlified.com/2008whats-youtube-doing-with-itunes-and-amazon-mp3/