This WordPress plugin allows the owner of a blog to change and test new WordPress themes without worrying about their visitors seeing odd things happen on the blog.
Once activated a new page called “Theme Tester” is linked from the Design page where you can activate the test. While activated you, as an administrator, can change themes as many times as you want without affecting what themes you regular visitors see.
Anyone with “switch_themes” capability will see the new theme however, but those users are going to be administrators on your site anyway. Not anonymous Internet users.