It seems, as interesting as the subject of using LED's as light detectors goes, they are not practical for anything more than simple on/off detection. Possibly ultra cheap, low part count, bidirectional communication. I remember an EDN design idea that used a 8-pin micro to both send information, and recieve it via the same LED. But ultimately...unless you're into mass production, the cost of a phototransistor is nothing
Also, you measured the voltage across the LED. Most designs these days actually reverse bias the LED, charging up its tiny capacitance, then measuring how quickly this discharges through a high impedance load (like a CMOS input). In effect, forming an oscillator. The frequency of which is dependent on the amount of light (and wavelength) the LED see's. Absolute measurement of the voltage across the LED, given the tiny currents involved could prove tough, as noise will be a big problem.
Its easy with microcontrollers, as well as FPGA's, since you have bidirectional I/O's, and it can all be done digitally.
Blueteeth