in Unity 3.4 it appears that the accelerometer (iOS) suffers from lag of 2-4 seconds. i did not notice this lag with the previous version of unity. I think this is a bug.
To test download the Roll-a-Ball project and test with the player → http://unity3d.com/support/resources/files/Roll-a-Ball.zip
@amcneilly, please mark your own answer as accepted - click the "check" button below the down voting thumb. The answer will appear in nice green and signal to the others that this problem was solved.
– aldonaletto