add_filter('simplemodal_login_form', 'mytheme_login_form');
function mytheme_login_form($form) {
    $users_can_register = get_option('users_can_register') ? true : false;
    $options = get_option('simplemodal_login_options');

    $output = sprintf('
<form name="loginform" id="loginform" action="%s" method="post">
    <div class="title">%s</div>
    <div class="simplemodal-login-fields">
    <p>
        <label>%s<br />
        <input type="text" name="log" class="user_login input" value="" size="20" tabindex="10" /></label>
    </p>
    <p>
        <label>%s<br />
        <input type="password" name="pwd" class="user_pass input" value="" size="20" tabindex="20" /></label>
    </p>',
        site_url('wp-login.php', 'login_post'),
        __('Login', 'simplemodal-login'),
        __('Username', 'simplemodal-login'),
        __('Password', 'simplemodal-login')
    );

    do_action('login_form');

    $output .= sprintf('
    <p class="forgetmenot"><label><input name="rememberme" type="checkbox" id="rememberme" class="rememberme" value="forever" tabindex="90" />%s</label></p>
    <p class="submit">
        <input type="submit" name="wp-submit" value="%s" tabindex="100" />
        <input type="button" class="simplemodal-close" value="%s" tabindex="101" />
        <input type="hidden" name="testcookie" value="1" />
    </p>
    <p class="nav">',
        __('Remember Me', 'simplemodal-login'),
        __('Log In', 'simplemodal-login'),
        __('Cancel', 'simplemodal-login')
    );

    if ($users_can_register && $options['registration']) {
        $output .= sprintf('<a class="simplemodal-register" href="%s">%s</a>',
            site_url('wp-login.php?action=register', 'login'),
            __('Register', 'simplemodal-login')
        );
    }

    if (($users_can_register && $options['registration']) && $options['reset']) {
        $output .= ' | ';
    }

    if ($options['reset']) {
        $output .= sprintf('<a class="simplemodal-forgotpw" href="%s" title="%s">%s</a>',
            site_url('wp-login.php?action=lostpassword', 'login'),
            __('Password Lost and Found', 'simplemodal-login'),
            __('Lost your password?', 'simplemodal-login')
        );
    }

    $output .= '
    </p>
    </div>
    <div class="simplemodal-login-activity" style="display:none;"></div>
</form>';

    return $output;
}