A very popular theme on WordPress.com is Sandbox. Why? Because (for security reasons), WordPress μ does not allow template editing, to prevent all types of malicious scripting. However, with the release of WordPress 2.5 and the shortcode system, I’ve been thinking that someone should make a templating system that could be used on MU without a risk of cross site scripting.
Instead of being a plugin, it would probably need to be a theme to prevent the “WTF” response when either your custom template is destroyed when you change a theme, or when you change a theme and your old custom one shows up.
I think I might be able to attempt creating this system, however I won’t need to if someone has already released something similar.