http://www.googleartproject.com/

See
http://m.lifehacker.com/5748796/google-art-project-walks-through-global-art-museums-street+view+style

It's even launched with a banner ad on the home page:
http://i51.tinypic.com/11769ut.png

Also http://googleblog.blogspot.com/2011/02/explore-museums-and-great-works-of-art.html

on Tue 1st Feb