Dave Grohl of the Foo Fighters was the guest host on Jimmy Kimmel Live and Kristen Bell was there (dressed as Magnum PI for Halloween). Grohl, dressed as Letterman, told her that if they jammed on a song together it would make him seem so cool to his daughters.
So Bell said yes and the rest is history. The did a awesome rock/metal version of 'Snowman" and that some how transformed into Metallica. It was fantastic.