http://en.wikipedia.org/wiki/Infrare...igital_cameras

Explains in detail why it'll be very hard to use the N900 or any other small portable device camera for IR photography.

Basically, IR light interferes so much digital photography that it must be blocked from entering the sensor in the first place. The fact that people can do IR photography with digital sensors is due the fact that the IR bocking was not always sufficient, like in some first gen SLRs.

Today, the IR filtering is much effective and more controlled. For doing IR photography with cameras like the one in N900, you would need to physically remove the IR filtering layer from the sensor and replace it with a visible-light filter. Also the optics aren't meant for IR frequences and don't work optimally with the software even if you could do the physical modifications necessary.