registering new jQuery after wp_deregister_script() not working

When setting your dependencies they need to be in an array.

wp_register_script('jquery', 'http://code.jquery.com/jquery-1.7.min.js', '1.7', 1);
wp_register_script('modernizr', get_bloginfo('template_directory').'/js/libs/modernizr-2.0.6.min.js', array('jquery') );    
wp_register_script('plugins', get_bloginfo('template_directory').'/js/plugins.js', array('jquery'), '', 1);    
wp_register_script('custom', get_bloginfo('template_directory').'/js/script.js', array( 'jquery') , '', 1);