I also tried WP plugin Theme Switcher (http://bit.ly/JmDfLL) and I was hopeful it would work. But the theme which is currently selected effects how the Widgets section in the WP Admin works. At least in my tests for 3.3.2.
I also ran across Alternative Theme Switcher (http://bit.ly/JXEw1e) which looked prommissing but hasn't been updated since 2009 so I didn't try it.
Seems like an obvious need as there are so many posts requesting such a feature. Everything is focused on the developer or admin users being allowed to choose a theme for testing before launching. As well, there are several plug-ins available for switching language, font size or based on user-agent.