Bill's code resulted in having the <span></span> be included in the id="title" which was problematic for the site I as working on my method breaks up the title a bit more to not including the <span></span> in the title attribute.

gistfile1.php

<?php
/**
 * Set a span around the letters "WP" in Site Title
 * @author Jason Tucker
 * Based on code by Bill Erickson
 * 
 * Why fork?
 * Bill's code resulted in having the <span></span> 
 * be included in the id="title" which was problematic 
 * for the site I as working on my method breaks up the 
 * title a bit more to not including the <span></span> 
 * in the title attribute.
 *
 * @param string $title full title
 * @param string $inside text
 * @param string $wrap html wrapper
 * @return string full title
 */
function jt_customize_site_title($title, $inside, $wrap) {
	$site_title = get_bloginfo( 'name' );
	$custom = str_replace('WP', '<span>WP</span>', $site_title);
    	$title = sprintf( '<h1 id="title"><a href="%s" title="%s">%s</a></h1>', trailingslashit( home_url() ), $site_title,$custom);
	return $title;
}
add_filter('genesis_seo_title','jt_customize_site_title', 10, 3);