I noticed this dark grey shadow at the bottom of my springboard after installing a custom theme (jailbroken 4s, redsn0w 0.9.15b1). Thought it was an element of the theme itself and ignored it but it started irritating me so I tried to find the png that might be causing it. That proved fruitless so I deactivated all themes, still the shadow persists, start in safe mode, still the shadow persists, last resort, restore to 5.1.1 using redsn0w 0.9.15b3 and STILL THE SHADOW PERSISTS!!!!! I suspect a physical fault with the screen but the shadow is only visible on the springboard, not in apps!! Please help!!! Here I've used a solid white background to illustrate what I'm talking about.