This is a fine idea. I would expand it a bit more to include native UI controls. I wish Apple would figure out a way to allow on the fly code changes without having to push an update. THAT would be nice.
I live and work in the UK, my designer friend lives on the other side of the world.
Whenever he has a new design for an app I'm making, he sends me the PSD files, I cut them up, export the images, add them into the app, compile and send him a new version of the app. Rinse & Repeat.
This is a crazy way of working. By allowing my designer friend to login to itheme.com, he can create different layouts of my app and test them in realtime.
I can then look at, and try out the different themes.