[daltore]

If they aren't COTS, you can make your own with commercially bought hall effect sensors (they look like transistors, just google "magnetometer" and you'll get results). With the individual sensors themselves being so small, you could actually probably place two of them close enough together to get a directional output (because you could measure rising edge and falling edge by doing some subtraction of the two voltages, i.e. if(hall1-hall2 < 0) direction=forward) It's a little more complicated than that obviously, because timing is so important, but given some thought, it should be fairly simple. The sensor cells are a couple bucks a piece.