What can you do if your WP7 accelerometer is not correctly calibrated? As I found out, there seems to be no OS mechanism to calibrate the accelerometer. So if you have a production device you will have to return it for repair or replacement. But what do you do if you have a loaner preproduction device like I do? Well, you have to fix the accelerometer the only way you can: through software.
A correctly calibrated accelerometer should return the vector (0, 0, -1) when you place your device with the screen up on a flat surface (like your office floor). If you have a skewed accelerometer like I do, then you should be able to compensate the difference to the reference vector, once you know what your device is reporting when placed on the floor with the screen up.
To do just that, I wrote a very simple Silverlight application for WP7 that polls the accelerometer every second and displays the vector components, both the sampled value and an average value in order to smooth out the natural variations in the sampling.
You can get the sample application with source code here. After running this app on the floor, I noticed that the Y component of the accelerometer vector was slightly skewed to -0.15, while all other components reported the expected values. After adding 0.15 to the Y component, I got back a calibrated accelerometer.
Now I can get back to my XNA gravity ball code and make it work like it should.