http://leaves-and-love.net/how-to-improve-wordpress-seo-with-schema-org/ (only works with WordPress SEO versions 1.7.1 and older!)

wpseo-schema-meta-function.php

<?php 
function yourtheme_meta_schema()
{
	if( class_exists( 'WPSEO_Frontend' ) )
	{
		global $wpseo_front;

		$canonical = $wpseo_front->canonical( false );
		echo '<link itemprop="url" href="' . esc_url( $canonical, null, 'other' ) . '" />' . "\n";
		$metadesc = $wpseo_front->metadesc( false );
		echo '<meta itemprop="description" content="' . esc_attr( strip_tags( stripslashes( $metadesc ) ) ) . '" />' . "\n";
	}
}
add_action( 'wp_head', 'yourtheme_meta_schema' );
?>