I did the same exact thing to my ipod
touch two weeks ago!
And my screen is the same exact way as you described yours is.
I took it completely apart, and discovered that the source of the discoloration was due to a stain inside the internal screen, it had been stained. It was a piece of Plexiglas,and i tried to scrub the stain out, with little luck, and when i put it back together the discoloration was still there.
I'm sorry to tell you that it's going to stay that way:(