RFID is about the only thing that springs to mind. The TI TIRIS stuff may do the distance you require. From my recollection, it uses around 300khz to send to the device. The distance depends on the antennas(coils of wire) you use. The theft detection systems used in shops works on detecting a resonant circuit in it's field. No data is transferred. The flat panel thingys you walk through are big coils of wire acting as the antenna.

Now, if you want close-range distance sensing , that's another story..... I've also been working on a brightness dimmer for ELV lamps lately.......help! I'm slowing down fast!!!!!