wp_authenticate wordpress 2.5

I was using the HTTP-Authentication plugin (http://wordpress.org/extend/plugins/http-authentication/#post-141) on 2.3.3 just fine. Upon upgrading to 2.5, it stopped working correctly.
It was as if wp_authenticate() was not firing. Does anyone know why this might not be working in 2.5?

I have included the meat of the code below:

if (! class_exists('HTTPAuthenticationPlugin')) {
	class HTTPAuthenticationPlugin {
		function HTTPAuthenticationPlugin() {
			if (isset($_GET['activate']) and $_GET['activate'] == 'true') {
				add_action('init', array(&$this, 'init'));
			}
			add_action('admin_menu', array(&$this, 'admin_menu'));
			add_action('wp_authenticate', array(&$this, 'authenticate'), 10, 2);
			add_action('wp_logout', array(&$this, 'logout'));
			add_action('lost_password', array(&$this, 'disable_function'));
			add_action('retrieve_password', array(&$this, 'disable_function'));
			add_action('password_reset', array(&$this, 'disable_function'));
			add_action('check_passwords', array(&$this, 'check_passwords'), 10, 3);
			add_filter('show_password_fields', array(&$this, 'show_password_fields'));
		}

		/*************************************************************
		 * Plugin hooks
		 *************************************************************/

		/*
		 * Add options for this plugin to the database.
		 */
		function init() {
			if (current_user_can('manage_options')) {
				add_option('http_authentication_logout_uri', get_option('home'), 'The URI to which the user is redirected when she chooses "Logout".');
			}
		}

		/*
		 * Add an options pane for this plugin.
		 */
		function admin_menu() {
			if (function_exists('add_options_page')) {
				add_options_page('HTTP Authentication', 'HTTP Authentication', 9, __FILE__, array(&$this, 'display_options_page'));
			}
		}

		/*
		 * If the REMOTE_USER evironment is set, use it as the username.
		 * This assumes that you have externally authenticated the user.
		 */
		function authenticate($username, $password) {
			global $using_cookie;

			// Reset values from input ($_POST and $_COOKIE)
			$username = $password = '';

			if (! empty($_SERVER['REMOTE_USER'])) {
				if (function_exists('get_userdatabylogin')) {
					$username = $_SERVER['REMOTE_USER'];

					$user = get_userdatabylogin($username);
					if ($user and $username == $user->user_login) {
						// Feed WordPress a double-MD5 hash (MD5 of value generated in check_passwords)
						$password = md5($user->user_pass);

						// User is now authorized; force WordPress to use the generated password
						$using_cookie = true;
						wp_setcookie($user->user_login, $password, $using_cookie);
					}
					else {
						// User is not in the WordPress database, and thus not authorized
						die("User $username does not exist in the WordPress database");
					}
				}
				else {
					die("Could not load user data");
				}
			}
			else {
				die("No REMOTE_USER found; please check your external authentication configuration");
			}
		}

		/*
		 * Logout the user by redirecting them to the logout URI.
		 */
		function logout() {
			header('Location: ' . get_option('http_authentication_logout_uri'));
			exit();
		}

		/*
		 * Generate a password for the user. This plugin does not
		 * require the user to enter this value, but we want to set it
		 * to something nonobvious.
		 */
		function check_passwords($username, $password1, $password2) {
			$password1 = $password2 = substr(md5(uniqid(microtime())), 0, 10);
		}

		/*
		 * Used to disable certain display elements, e.g. password
		 * fields on profile screen.
		 */
		function show_password_fields($show_password_fields) {
			return false;
		}

		/*
		 * Used to disable certain login functions, e.g. retrieving a
		 * user's password.
		 */
		function disable_function() {
			die('Disabled');
		}