Folks at the Maemo.org have find a way to enable Bluetooth HID by default. This makes any bluetooth mouse and bluetooth keyboard to work with Nokia N900.
Usually in Nokia N900 bluetooth keyboard works well, but incase of mouse it moves very fast we cant easily point it out. Nokia N900 is open to anything so here they mod it to work with mouse.
Interested have a look at this thread to make it work for you too.