AI contestants have shown producers what is popular :Taylor Hicks sang Sam Cooke's "A Change Is Gonna Come" in his audition...and in 2008 Seal worked with David Foster to produce Seal's singing that song and Seal sang it for Obama along the campaign trail.
Jason sang Jeff Buckley's "Hallelujah" in 2008 and later in 2008 Il Divo, produced by Simon Cowell, has it on their new album and was making the talk show rounds singing it in Italian.
There are probably more examples of this but those are two I recently noticed.
Happy 2009!
P.S. Michael Johns released a Christmas song on itunes--album to come in 2009.