This video demonstrates Nerdkits' Theremin Halloween costume:
The costume features two homebuilt infrared distance sensors mounted on the wrists. Someone wishing to "play" your costume can move his or her hands up and down above the distance sensors. One hand controls the pitch of the note, and the other controls the volume. The sound is emitted from a piezoelectric buzzer mounted on the breadboard, worn around the neck.
The idea behind this costume is to recreate the function of a theremin and build it into a Halloween costume, giving someone that approaches you while trick-or-treating the ability to wave their hands and make music with your costume like a theremin.
DIY details are available at the Nerdkits site.