function devpress_login_form_shortcode() {
    if ( is_user_logged_in() )
        return '';

    return wp_login_form( array( 'echo' => true ) );
}

function devpress_add_shortcodes() {
    add_shortcode( 'devpress-login-form', 'devpress_login_form_shortcode' );
}

add_action( 'init', 'devpress_add_shortcodes' );