Edit Author Slug allows you to change the author slug of your users. (ie - http://example.com/author/username/ can be changed to http://example.com/author/user-name/)
Version 0.2 adds checks for existing slugs to avoid duplicates, and adds properly sanitizes slug before comparing and commiting to database.
Plugin URL: http://brandonallen.org/wordpress/plugins/edit-author-slug/
WordPress Extend link: http://wordpress.org/extend/plugins/edit-author-slug/