I found a number of answers. There are a few plugins that can do it, or you can use a jquery style switcher. Apparently there’s also a php style switcher somewhere on github.
And here’s another thread I found on this same topic (a lot of these links are from there). http://themeforest.net/forums/thread/stylesheet-switcher-for-wordpress/35461?page=1
I’m almost ready to release my first theme here, and questions about building the demo site are now coming up.
What script or plugin do you use to create the theme options panel that displays on the demo site (usually as a fly-out panel)?