This is actually a wallpaper made for the iPhone, but for me it works beautifully on the iPad as well. It’s called simply Landscape 01 and it’s by Paul Isabella. I spotted it via the RSS feed from the superb Poolga.com site.
I generally use more striking (or personal) images for lock screen wallpapers, while going with very simple, non-distracting backgrounds for the home screen. This wall seems to strike the perfect balance for use on both screens. It’s a gorgeous, subtle image that works perfectly for me in both places.