The problem with this is that the N900 does not have a compass. It doesn't know which direction it is pointed, so it can't do an augmented reality type of sky display. While in principle one could do a calibration orientation of the phone, and then use the accelerometers to track how the position changes as a function of time, in practice that won't work because the accelerometers aren't very accurate. They are intended to be good enough to tell if the phone should be displaying in portrait or landscape mode. They are not nearly accurate enough to keep track of the phone's azimuthal orientation as it is slowly rotated.