If I don't have screen rotation lock turned on on my iPad 2 (OS 5.1) and am trying to use it in portrait mode, it's constantly rotating to the right when I don't want it to. Since it felt overly sensitive, I did a test to see how far it would have to tilt off of flat to rotate to a given direction.
To change rotation to up, down, or left, I needed to tilt it maybe 40 degrees off of flat. To change to right rotation, though, it only took a tilt of maybe 20 degrees.
I have the 'Clinometer' app (a fancy level), and its readings seem more or less accurate, so it doesn't seem like a problem with the accelerometer. Do others experience the same thing? Is there any setting I can change to tweak this?