The IR filter is properly inside the lens. Most small form factor cameras have the IR filer stuck in between the actual lens and a glass/plastic plate so it is nearly impossible to remove.

I would'nt recommend you doing it to the N900, but a think that you properly can do is to filter out everything except IR light. This can be done quite effectively by using a piece of developed film of a totally black subject. Then the camera's automatic light correction will properly be able to get some sort of image...