I did a quick ask-around to people who know much more about radio and electronics than me and the suggestion was that this idea might generate some data but not in any stable way.

An interesting link to follow might be this one:
http://en.wikipedia.org/wiki/Fluxgate_compass

So, to sum it up: electronic/digital compasses are available but to use the FM oscillator in the N900 to do it might prove hard. This would be on account of the FM-transmitters inability to act on really low frequencies.

So any data that would be registered would be overloaded on the FM frequency (as I understand it), in the MHz region, resulting in a violently spinning reading (in the region of 6000 RPM).