After apple sold a million Apple iPad and if you are one among them, here are few iPad Themes which you can download and use it for your iPad. Manav already wrote on some amazing source of wallpapers which you can find here.
Lost theme for the Apple iPad

iPad Wallpaper collection at Daily iPhone Blog

Clash of the Titans iPad theme :

Get complete details here on how to apply it.
Beautiful Lake and Mountain Scenory Wallpaper :

Lady Gaga theme for the Apple iPad
