Setup canonical links for WPEC products in multiple categories.

gistfile1.php

<?php

/**
 * Fix canonical for posts in multiple categories
 */
class CanonicalCategoryFix {
  
	function CanonicalCategoryFix() {
		add_filter( 'wpseo_canonical', array( $this, 'canonical_permalink' ) );
		add_filter( 'wpsc_change_canonical_url', array( $this, 'canonical_permalink' ) );
	}
	
	function canonical_permalink( $canonical ) {
		global $post;
		
		if ( is_single() && get_post_type() == 'wpsc-product' ) {
			add_filter( 'wp_get_object_terms', array( $this, 'wp_get_object_terms_single' ) );
			$canonical = get_permalink( $post->ID );
			remove_filter( 'wp_get_object_terms', array( $this, 'wp_get_object_terms_single' ) );
		}
		return $canonical;
	}
	
	function wp_get_object_terms_single( $terms, $object_ids = null, $taxonomies = null, $args = null ) {
		if ( is_array( $terms ) && count( $terms ) > 0 ) {
			return array( $terms[0] );
		}
		return $terms;
	}
	
}
$CanonicalCategoryFix = new CanonicalCategoryFix();

?>