If register_sidebar() had a parameter for a capability and these registered capabilities were checked during rendering of /wp-admin/widgets.php, it would be possible to allow editing of certain sidebars only to certain users - great for a site with several people responsible for editing different areas.
Personally, I'd use this by setting up some sidebars as admin user only to help me build a client's site, but happily let them have access to other sidebars of text widgets (for example).
I tried writing a plugin to do this and it all goes well until you try to change the sidebars that your user can see. Then the AJAX triggers retrieve_widgets() and all your hidden sidebars get deregistered and emptied into the inactive widgets holder :-(
So I guess I don't just want the capability parameter, but for /wp-admin/widgets.php to not mess it up.