its theme controlled, until themes are out you cannot change this. once themes are out its up to the theme developer to allow the theme to use the home screen image or force one like the current theme does.
This is really a silly issue... how hard can it be to just use the home screen background as the app screen background? I am sure RIM has bigger fish to fry than this but for someone who has OCD like me... the ugly blue background really clashes with my backgrounds! :] ha