function rss_post_thumbnail($content) {
    global $post;
    if(has_post_thumbnail($post->ID)) {
        $content = '<p>' . get_the_post_thumbnail($post->ID) . '</p>' . get_the_content();
    }
    return $content;
}
add_filter('the_excerpt_rss', 'rss_post_thumbnail');
add_filter('the_content_feed', 'rss_post_thumbnail');

Published on June 14, 2011