I remember seeing something about this before. What carrier version of the phone do you have, and what version of Android? Are you rooted? On the stock ROM, or a custom one?
IIRC, it's related to the ripple effect, and only visible on dark backgrounds. Do you have ripple effect turned on? If so, turn it off and see if that fixes it. If your lock screen wallpaper is dark, try a light one and see what happens.
I *think* this is fixed on the latest versions of Android.