I just rewrote John O'Nolan's UI Labs plugin. I think it would be more WordPress way to add CSS file admin head

ui-labs.php

<?php
/*
Plugin Name: UI Labs
Plugin URI: 
Description: Experimental WordPress admin UI features, ooo shiny!
Author: John O'Nolan
Version: 1.0
Author URI: http://john.onolan.org
*/

function ui_labs_css() {
	$url = plugins_url('/ui-labs.css', __FILE__);

    wp_register_style( 'ui_lab_css', $url, false, '1.0' );
    wp_enqueue_style( 'ui_lab_css' );    
}

/**
 * To add CSS files on specific page in wp-admin, instead
 * admin_enqueue_scripts use admin_print_styles-$page hook
 */
add_action( 'admin_enqueue_scripts', 'ui_labs_css', 1000 );

?>