[bastian74]

It is an accelerometer. Holding the phone still does have an acceleration applied to it, the pull of the earth. If you throw the phone up and let it free fall the accelerometer will read zero on all 3 axis.

Download shocktester. It will print out acceleration in all 3 axis but will not show any rotation. The bubble level works by detecting the amount of acceleration in the horizontal plane, relative to the phone. A gyroscope would make a terrible level.

And a bubble level is in and of itself an accelerometer. If you hold one level and accelerate horizontally you'll see the bubble move towards the direction of acceleration.