Darrel of Fort Collins, CO, wrote in with his demo of an Android app that controls servos via audio.

This is a demo of an android app that directly controls servos via a special audio tone from your android phone. No micro-controller or electronic components of any kind are needed aside from a battery.

If you would like to try this, connect 3 to 5v to the + and – of your servos, connect the left and right channels of the headphone cord to the signal lines, and connect the headphone ground to the power ground. Do not try this if you do not have some idea what you are doing or you may damage your phone! Also if you get the + and – backwards on a servo header it will burn up the servo.

More info at cellbots.com and code on Google Code.