TriCorder yup sounds like a killer app to be sure.

We could even hack a compass into your fm transmitter-receiver
notion - I am quite busy with other things but this sounds like
it could be a game-changer for the n900 usage.

I have my spare disassembled and am looking for space to
put a coil for just such a hack - looks like the only site would be
near the stylus insertion hole, which suggests an idea.
If we replaced the stylus with a long air coil it might be
possible to wire that into the FM side and get some interesting
results from ambient fm noise (I do work on ambient noise
data acquisition techniques in geophysics so it fits right in)
Any compass app would need to integrated with accelerometer
data to provide an orientation basis.

All this might hinge on being able to (quickly) toggle FM
transmit and receive functions (which may be too slow for interaction detection)
but a passive coil on receive only might at least be useful
for detecting metal as the induction change from nearby
ferro-magnetics might be detectable by noise attenuations.