This looks like an interesting concept which was just launched yesterday: Livethemer, a way to change the layout and style of your site, almost the way you can in a DrupalGardens site. The service requires a module, an account on livethemer.com is optional. According to the site, the team worked for almost 2 years on this project. While I don't believe you should give clients this kind of theming power, and professional themers won't use this either, there might be a niche target group for it. Check out a demo video on vimeo.