I replaced lines 1954-1955 in sitemap-core.php to the following so that it will use the latest comment date as the last modified date if it is newer than the post's last modified date. One caveat: I am using memcached with wordpress so most of these comment queries are cached and do not go to the database.
$lastModifiedDate = $this->GetTimestampFromMySql(($post->post_modified_gmt && $post->post_modified_gmt!='0000-00-00 00:00:00'?$post->post_modified_gmt:$post->post_date_gmt));
$latestComment = get_comments('post_id='. $post->ID .'&number=1');
if(count($latestComment)) {
$latestCommentDate = $this->GetTimestampFromMySql($latestComment[0]->comment_date_gmt);
if($latestCommentDate > $lastModifiedDate) {
$lastModifiedDate = $latestCommentDate;
}
}
//Add it
$this->AddUrl($permalink,$lastModifiedDate,($isPage?$cf_pages:$cf_posts),$prio);