I've seen an application using an LED and a pair of I/O pins on a micro controller that allows a single LED to work both as a light source, and a detector. Basically when the LED is reverse biased the capacitance of the diode charges, when you yank the power it discharges and the micro controller times the length of the discharge. Giving you at least the ability to use low baud transfers (300-2400baud or so) using only 1 extra I/O pin on an MCU and no extra external components. I can't test this for probably another month as I don't have my scope or breadboard set up where I'm at, but was hoping someone else had tried this. I do know that LED's are generally most sensative in the part of the spectrum immediatly bellow the colour that they emit when forward biased.