I've been meaning to try this for sometime now. Surf to this page with your iPhone and click and hold on the image you'd like then save to your photos. From there you'll find it in your camera roll where you use as wallpaper! If you want to make your own, here is a CS3 photoshop template
(645Kb) I created that also has a guide layer to help centre your design.
Labels: Graphic Design, iPhone, Logo, Wallpaper