iPhone 3G: /System/Library/CoreServices/SpringBoard.app/N82AP.plist
iPod Touch 2G: iPhone 3G: /System/Library/CoreServices/SpringBoard.app/N72AP.plist
Multi-tasking
Wallpaper
Battery Percentage
In my experience, wallpapers slowed down my device a lot! The reason for this is the way Apple has implemented the wallpapers, within some pointless overlays. To fix this, simply delete the following files from \System\Library\CoreService\SpringBoard.app:
WallpaperGradientLandscapeBottomT.png
WallpaperGradientLandscapeTopT.png
WallpaperGradientPortraitBottomT.png
WallpaperGradientPortraitTopT.png
WallpaperIconDockShadow.png
WallpaperIconDockShadowT.png
WallpaperIconShadow.png
WallpaperIconShadowT.png