When I was pregnant, a girlfriend gave me a cream (the brand was Dermologica) to fade dark spots. I believe you can probably find something similar at a salon that has skin care products. However, I don't know that this really helped all that much. Maybe if you used it more religiously than I did . . .
My baby is now almost 17 months old and that line has finally faded out, although I can still see it just barely. My doctor told me that it would never entirely go away, but I think a lot may have to do with your genetics and coloring (i.e., I would imagine a person who has very pale skin would have a harder time getting that line to fade).