Rock Band 2 drum kit hides mystery port

The updated drum kit for Rock Band 2 apparently has a mystery port located in the back. This port, which was speculated to be used for a second kicker, won't actually be used for that. This, of course, has led to all kinds of speculation about what it's function actually is. Could it support something new, like the Guitar Hero rumor we heard awhile back? Harmonix isn't saying, for now.

[Via Engadget; image courtesy of Joystiq]