To remove this gradient/shadow, all we have to do is put up a folder like this on our Winterboard theme folder: "No gradient/Folders/SpringBoardUI.framework/WallpaperGradientPortraitFull@2x.png" (for the iPhone 4) or "No gradient/Folders/SpringBoardUI.framework/WallpaperGradientPortraitFull.png" (for all other iDevices).
Both the PNGs should be transparent, with 1x960 or 1x480 accordingly to the device.
A reader sent me that fix and demonstrated the difference. I wish I could get this to work.