as much as i'd love to see something like this, i really don't think they should do it. you can guarantee that 90% of the themes would be work ripped off deviantart and other similar websites and then sold on. i would not support that at all. cropping and shrinking someone else's work to make look good on the iphone is not something someone should be able to make a living off, especially when that work is owned and copywriter by someone else.
i know there are people out there who could and would make entirely original themes, background and icons created from scratch, but there would be many more that would grab the background and icons from somewhere else and simply package it as their own.