Hi TiagoTiago,

afaik the IR module in the N900 only supports the sending of IR Signals, not receiving them (HW limitation).
You can use it as a remote for TVs for example, but not for communicating with other devices.
Using the camera to pick up the IR beam is also out of discussion, as it looks in the wrong direction (otherwise you could use it to detect how large/bright the IR-light "blob" is and measure the distance that way. But this would be really inaccurate).

Your second question goes in the same direction.
Metal detectors usually use a very distinct electromagnetic field. The direction is exactly known. N900s FM Transmitter emits also an electromagnetic field, but it will spread in multiple directions and measuring it with the FM receiver will yield to nothing.
Theremins work by measuring the field capacity. Your hand is like the second plate of a capacitor.

Sorry to disappoint you.