I have found that when the mouse is grabbed and I scroll the mouse wheel, Mouse.getDY() is returning values.
It seems if the mouse speed is over 100 then the DY will not be zero so in game camera will move up and down if you dont scroll one "notch" at a time.
I don't know if this is cross platform but I am on OS X with both Java 6 and Java 7.
What is happening here?