This is how to dim your wallpaper or remove dim on ios 4.0
What You need:
Jailbroken
iOS 4.0
SSH,iFunBox or something like that
a editor that supports Transparency
1. locate your systems WallpaperGradientPortraitFull.png
it should be under /System/Libary/PrivateFrameworks/SpringBoardUI.framework
2. iPhone/iPod Touch WallpaperGradientPortraitFull.png
2. iPad WallpaperGradientPortraitFullT.png and WallpaperGradientLandscapeFullT.png
3. then you need to make a new layer and fill it black or a color of your chose then erase the background layer and change the new layers transparency save it as a png and save it to your system in that directory WINTERBOARD CANT DO IT FOR YOU
4. I FORGOT RESPRING!!!!
Pics
Before

After (i didnt make mine that much darker ^^)