If you're just interested in audio signalling there's this on the transmit side: Arduino Playground - DTMF
A while back I played with HC-SR04 ultrasonic range finders to do point to point communications and had rudimentary FSK and PSK code working at very short ranges just using square waves driving the transducer. Problems at range were likely due to echos or the resonant properties of the transducer. A PPM scheme worked at a couple meters, but throughput was limited to maybe 100 baud because of echo persistence.