A nifty, new Theme Customizer has just been added to WordPress.com. To try it out, click the "Customize" links on your Appearance → Themes pages. You can customize any theme before activation, including premium themes. Blog announcements and support docs are in the works.
We know you are awesome and will have questions, and you may even find some bugs! Please post 'em here, and we'll keep a close eye on things for you as we get started with this totally neat new Customizer feature which will also be launched in the upcoming core WordPress 3.4 release.