After much head scratching and nudges from others I found the reason why the sliders and the dropdown menu stopped working in the backbone theme when I upgraded to WP3.5
WordPress 3.5 adds a different backbone.js to core:
in theme-js.php you have:
wp_enqueue_script( 'backbone', trailingslashit( get_template_directory_uri() ) . 'includes/js/backbone.js', array('jquery') );
That never gets loaded, presumably as it conflicts with the WP core.
I renamed the themes backbone.js file and edited that line:
wp_enqueue_script( 'backbonetheme', trailingslashit( get_template_directory_uri() ) . 'includes/js/backbonetheme.js', array('jquery') );
All is now working again….