LED as transmitter and photodetector in the same circuit?
Using an LED as detector as well as a transmitter was invented by Forrest Mimms back in the late 60s / early 70s.  His Wikipedia page has some info on LED based photo-detectors.  Here is a more recent Makezine article which he wrote about it.

As for a way to switch between LED modes, you could just use two low leakage SPDT analog switches to select between forward biasing the LED for transmitting and connecting it up reversed biased to an op amp as a receiver.

Since LEDs are so cheap, the best solution might be using two LEDs at both ends of the link and just always have the receiver circuits active.  In this case, you could even use two different color LEDs and have a full duplex link.