When you set background for your mobile many sections of the display is taken up by taskbar or icons. Jumba @ XDA has developed an application , I C Thru U, which crops the wallpaper you might have already applied and then set tem as background of taskbar and soft key bars in your Windows Phone. This makes them look transparent.
Figure the difference between second and third image and you will know how it will be on your windows phone. You can download it from XDA Forum from here.