IR shooting is impossible. It is blocked by special physical filter in camera. It is needed for correct white balance in visible range. Cameras from Sony can mechanically remove it and use IR light, more light. IR flash is not important, more light is important.
It is possible to make IR shots with usual camera without removing filter, by adding black filter. But then you have get 10-12 steps darker images, so this is not suitable for night shooting.

Night mode in n900 just means, that now you can shoot crap at night. It allows you to set very high ISO and long exposure which would result in very noisy and blurred image.