category-slug based single post template for WordPress

//creates category-slug based template for single post
add_filter('single_template', create_function(
    '$the_template',
    'foreach( (array) get_the_category() as $cat ) {
        if ( file_exists(TEMPLATEPATH . "/single-{$cat->slug}.php") )
        return TEMPLATEPATH . "/single-{$cat->slug}.php"; }
    return $the_template;' )
);