ThemePerPost is a WordPress plugin that lets you change your blog’s theme for individual posts or pages
ThemePerPost is ideal for people who want to style their about page, create a unique landing page, show theme previews for design showcases, split one WordPress install into multiple sub-sites. It’s for anyone who wants to have posts and pages with different styles.
All you have to do to change theme for one or more posts or pages is upload the theme you want to use to the usual WP theme directory add a Custom Field to the page/post you want to style.
See the Creating a Theme tab for details of how to create a new theme and to use an existing one.
More Info: http://www.steveify.com/themeperpost/