You need an external keyboard and mouse (usually) to keep it from going to sleep in clamshell mode.
InsomniaX should work as well.
WAIT!
I just remembered that an Apple Remote can wake up a laptop in clamshell mode.
That gives you VLC, Quicktime, and Front Row controls.
I'm happy to report that no external keyboard or Apple Remote was needed. The bluetooth mouse worked on its own to wake up the computer and use in clamshell mode.