// Get a Zend_Auth instance
$auth = Zend_Auth::getInstance();

// Setup adapter
$adapter = new Zend_Auth_Adapter_DbTable($dbAdapter, 'users_table', 'username_column', 'password_column', 'SHA1(?)');

// Setup user provided username and password
$adapter->setIdentity($username)
            ->setCredential($password);

// Authenticate
$result = $auth->authenticate($adapter);

$result->getCode(); // Zend_Auth_Result::SUCCESS
$result->getIdentity(); // $username
$result->getMessages(); // array() unless Zend_Auth_Result::FAILURE*

if ($result->isValid()) {
    // Set user row object as the identity
    $auth->getStorage()->write($adapter->getResultRowObject());
}

$identity = $result->getIdentity();
/*
$identity->username;
$identity->password;
$identity->email;
*/