"it could be due to the screen being hit repeatedly, because i know my ipod touch had a white line across the screen after it was dropped on the screen. the thing i would recommend doing is going to your local Apple store and asking them if they would fix it."
Here is a website where someone got water and or drips of water on the screen which can result in getting into the iPhone. Has this happened to you?
I just saw at the top of this discussion - This solved my question - "Go to Settings -> General -> Accessibility . In section Physical & motor set the Switch Control to OFF. Blue lines are not issue."