When did Google start selling music?

Update: ajpresto pointed me to a link from the Google blog that this music search began a year ago. I still wonder how iTunes seems to continue to maintain the number one link each time.

Jimmy sent this link over to me this morning. Here is a screenshot of the part to discuss:

As you can see when you search for this band and their song, you get a cover art shot plus links to buy the song on iTunes or Napster. This appears to have been launched in early 2006. When you click the "More music results" you go to a page that looks like the below which includes links to iTunes, Napster, Rhapsody and Walmart Music.

A search for Britney shows 9 stores selling her first album:

When searching for U2 and clicking "all 11 stores" from the first link, here is the result. As you can see iTunes Music Store shows up first again. There certainly seems to be a defined order for the store results.

So what exactly is this? It sure seems pretty intense overall and each time I searched the iTunes Music store came up first. Does this mean that Google will be selling these spots as "advertisements" in the long run? Or just that a relationship with a company pushes you up the list? Will anyone click links that say "more stores" or will they just purchase from whatever is shown to them? And how far are we from Google pushing their Checkout service on these same pages?