There are 2 files to remove the shadow of the wallpaper and dock icons...
1.
WallpaperIconDockShadow@2x.png
2.
WallpaperIconShadow@2x.png
You can deduce from the name of the files which one is which. By the way there are in the springboard folder. Since you are using 3GS, I would SSH into that folder and copy the icons above (copy the ones that do not have @2x). Create them as new transparent files and re SSH them back. You should be set. That is what I did for my iPhone4.