I’ve liked this idea for some time, and would like to see more themes using it to handle features. Frank at WP Engineer has an example function showing how a function could activate a plugin in someone’s WordPress install. If a theme or themes are using features that can be abstracted out and act as a plugin, this would be one elegant way to activate said plugin when a user wants the feature up and working.
Take a look at TGM Activation. It’s a php class that you can add to your theme that handles all this for you. All you have to do is pass it an array of plugins (either in .org repo or inside the theme).