Beginning tonight, Google’s new enhanced music search results will feature promotional songs from record labels, opening the door for Google to treat music search as a paid-content opportunity rather than just the shortest path to a song. Partner site Lala.com revealed last week that it would feature exclusive content in search results, and is set to announce tonight that about 20 artists will offer exclusive promotional songs on Lala with featured placement in Google search results. Artists involved include Mos Def, Arctic Monkeys, Green Day, Lady Gaga and Norah Jones.

The inclusion of promo material treads uncomfortably close to the line between neutral search and sponsored content. Rather than simply delivering an artist’s most popular songs in response to a query, Google is allowing someone else to hand-pick at least some of the songs that appear in its results — a small but real betrayal of user trust. The video Google released to introduce its Discover Music program last week even notes that a search for Green Day brings back “links to Green Day’s most p