I recently had to authenticate against an outside server for Registered User content access. This isn't a huge amount of code but hopefully this will help someone save some time.

// Check to ensure this file is included in Joomla!
defined('_JEXEC') or die();

jimport( 'joomla.plugin.plugin' );

class plgAuthenticationWebservice extends JPlugin
{
	/**
	 * Constructor
	 *
	 * For php4 compatability we must not use the __constructor as a constructor for plugins
	 * because func_get_args ( void ) returns a copy of all passed arguments NOT references.
	 * This causes problems with cross-referencing necessary for the observer design pattern.
	 *
	 * @param object $subject The object to observe
	 * @param 	array  $config  An array that holds the plugin configuration
	 * @since 1.5
	 */
	function plgAuthenticationWebservice(& $subject, $config) {
		parent::__construct($subject, $config);
	}

	/**
	 * This method should handle any authentication and report back to the subject
	 *
	 * @access	public
	 * @param   array 	$credentials Array holding the user credentials
	 * @param 	array   $options     Array of extra options
	 * @param	object	$response	Authentication response object
	 * @return	boolean
	 * @since 1.5
	 */
	function onAuthenticate( $credentials, $options, &$response )
	{
		
		require_once(JPATH_LIBRARIES.DS.'phpxmlrpc'.DS.'xmlrpc.php');		
		
		$message = '';
		$success = 0;

		if (strlen($credentials['password']) && strlen($credentials['username']))
		{
			$client = new xmlrpc_client("/servlet/XmlRpc", "somemachine.somedomain.com", 80);

                        // let client give us back php values instead of xmlrpcvals
			$client->return_type = 'phpvals'; 
			$xmlrpcmsg = new xmlrpcmsg('some.service',
                                     array(php_xmlrpc_encode($credentials['password'],
                                     php_xmlrpc_encode($credentials['username'])));
			$response =$client->send($xmlrpcmsg);
			
			$success = (boolean)$response->value();
		}
		else  {
			$message = 'you need to submit a username and passcode!';
		}

		if ($success)
		{
			$response->status 	     = JAUTHENTICATE_STATUS_SUCCESS;
			$response->error_message = '';
			$response->fullname = "some users fullname"; //might not need this
			$message = 'Access granted';
		}
		else
		{
			$response->status 		= JAUTHENTICATE_STATUS_FAILURE;
			$response->error_message	= 'Failed to authenticate: ' . $message;
		}
	}
}