download healthcheck from the repos and swipe right until you get to "Sensor Explorer". Go in there and click on the n900.accelerometer. Then click "Start" with your phone laying on a flat surface that you know is *roughly* level. The output from the accelerometer should be approx 0 for both x and y axes and approx 10 for the z-axis.

If not, then you may have a borked accelerometer.