Use this class to create a plugin that will send an email when a post goes into pending review status. It should be noted that auto-save has to be turned off, if not multiple emails will be sent. The class handles turning off auto-save.

pending_review_notifier.php

class PendingReviewNotifier
{
        private $_email_sent_flag = false;
	
	public function run()
	{
		// add notification action to save_post hook
		add_action('save_post', array(&$this, 'send_pending_review_notice'));

		// add disable autosave hook
		add_action('wp_print_scripts', array(&$this, 'disableAutoSave'));
	}

	public function disableAutoSave(){
		wp_deregister_script('autosave');
	}
	
	public function send_pending_review_notice($id) {

		// the save_post hook is called twice on every post save, so I'm getting two emails
		// I need to set a flag to check if an email has already been sent
		if(empty($this->_email_sent_flag)) {
			$post_status = null;
			if(!empty($_POST['post_status'])) {
				$post_status = $_POST['post_status'];
			}
			
			$user_id = null;
			if(!empty($_POST['user_ID'])) {
				$user_id = (integer)$_POST['user_ID'];
			}
			
			$user_data = null;
			if(!empty($user_id) && is_int($user_id)) {
				$user_data = get_userdata($user_id);
			}

			//verify post is not a revision
			if ( !wp_is_post_revision( $post_id )  && !empty($post_status) && $post_status == 'pending') {
				$message = 'A post was placed in pending review status on blog: ' . get_bloginfo('name').PHP_EOL.PHP_EOL;
				$message .= "Post Title: " . $_POST['post_title'] . PHP_EOL.PHP_EOL;
	
				// Choose one of the following options to show the author's name
				$message .= "Post Author: " . $user_data->display_name . '('.$user_data->user_email.')'. PHP_EOL.PHP_EOL;
				//$message .= "Link: " . get_permalink($id);
	
				$subject = "Post in pending preview status: " .$_POST['post_title'];
				
				$recipient = get_bloginfo('admin_email');
				
				wp_mail( $recipient, $subject, $message );
				
				// set the sent email flag
				$this->_email_sent_flag = true;
			}
		}
	}
}

// start
$pending_review_notifier = new PendingReviewNotifier();
$pending_review_notifier->run();