Updtaed for 3.3, thanks xavisys || WidgetWithBehaviorScript: Template for a WordPress widget which enqueues an accompanying behavior script; script only output if widget is rendered.

WidgetWithBehaviorScript.widget.php

<?php

class WidgetWithBehaviorScript extends WP_Widget {
	
	function __construct() {
		parent::__construct(__CLASS__, 'Widget with Accompanying Behavior Script', array(
			'classname' => __CLASS__,
			'description' => "This WordPress widget serves as a pattern for how to enqueue a script only if the widget is actually rendered."
		));

	function widget($args, $instance) {
		// Enqueue the .js script which accompanies the widget
		wp_enqueue_script(
			__CLASS__,
			trailingslashit(get_template_directory_uri()) . __CLASS__ . ".js",
			array('jquery'), // Whatever you want
			filemtime(trailingslashit(TEMPLATEPATH) . __CLASS__ . '.js'),
			true // Must be true (in_footer)
		);

		extract($args, EXTR_SKIP);
		global $Shopp;
		
		echo $before_widget;
		echo $before_title;
		# ...
		echo $after_title;
		# ...
		echo $after_widget;
	}

	function update($new_instance, $old_instance) {}
	function form($instance){}
}

?>

Is there a similar trick, when handling an accompanying .css file reference?