As the very last bits of the credits are rolling, a very pretty, newly recorded, sans-lyrics version of the Firefly theme is played.
On the commentary, the last thing Joss says is to watch the credits....so I did. It was such a nice nod to the fans of the series to put it on there.
If anyone knows how to rip that music, I would love to have a file of it. Listen to it and see....it's very lovely.