i think from what you say that you want to use the N900 as a spectrum analyzer, and its a great idea to have something like that in your pocket and scan frequencies from sound to microwave (you missed the Mic) , but i hate to be the one to say it, it can not be done , its a hardware limitation, (for all except the audio and maybe FM if the SNR value can be read from the Internal FM receiver)
and let me explain why
first the N900 got only an infrared transmitter no receiver and second the best that can be done with WiFi is something like net stumbler that will draw a plot of the signal strength of the Detected WiFi devices,
a WiFi device cannot by any means scan a spectrum of frequencies like a real spectrum analyzer dose simply because it was not designed to have an interface or report the low level data of the radio receiver, most of the low level operations are handled by the WiFi Device's Firmware the same applies to the blue-tooth and GSM (2G and 3G)

i have interfaced GSM and blue tooth modules to micro controllers and built my own hardware for a car computer, and from all the data sheets and all the interfaces i have seen, most of the information that can be used to make something like this is handled internally inside the receiver itself and cannot be read from outside.