Callback to themes for dynamic sidebar function

I’m working on a theme where I want to add specific classes to the wrapper around each widget, depending on whether it is the first widget in the sidebar, second, third, etc. The reason I’m doing this is to get the widgets laid out using the Blueprint CSS framework.

For the time being, I’m handling this by creating a customized version of the dynamic_sidebar function and sticking that in my theme’s “functions.php”. This approach works, but is obviously brittle, and other theme developers might benefit from being able to customize their widgets’ presentation by position.

I propose passing the count of the current widget to the theme, and allowing for values returned by functions to be included in the register_sidebar(array(whatever)) Perhaps this is already possible and I’m missing it.