ソース紹介

function qrcode_output_func( $atts, $content = null ) {
	extract( shortcode_atts( array(
					'size' => '150',
					'errorlv' => 'L',
					), $atts ));
	if ( "" == $content ) {
		$content = get_permalink();
	}
	return '<img src="http://chart.apis.google.com/chart?chs=' . $size . 'x' . $size . '&cht=qr' . '&chld=' . $errorlv . '&chl=' . rawurlencode($content) . '">';
}
add_shortcode( 'qrcode', 'qrcode_output_func' );