I remember a while ago Haochi posted pictures from YouTube with that hinted at this (iTunes and AmazonMP3 integration) it seems to be rolling out now.
I just found it when checking out YouTubes april fools joke:
+ Show video
Screenshot: 
img371.imageshack.us/my.php?im ...
The button links to:
amazon.com/The-Greatest-Hits/d ...
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Previous on Google Blogoscoped: blogoscoped.com/forum/123573.h ...
Looks like they decided to have the buttons as separate files instead of using CSS sprite.
youtube.com/img/itunes.gif
youtube.com/img/amazon.gif
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I don't know if it's new but there are links to itunes stores from youtube artist personal page (like Rick Astleytv's). See the "buy album":
youtube.com/user/RickAstleytv
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