I'm guessing the the source of the problem is the same, ampersands or other special characters that haven't been HTML encoded. I also aggregate Raven's feed, so I can definitely confirm that problem is the same there.
Here's what I had to do to make my feed validate.
I'm using the most recent version, WordPress 2.0.4 released July 29, 2006. If you're using an older version the file names and line numbers may not match up exactly.
The PHP function htmlentities($string) converts any characters within the specified $string to the matching html encoded equivalent. If the character is already encoded it leaves it as is.
file: wp-rss2.php line 49
PHP Code:
<wfw:commentRSS><?php echo htmlentities(comments_rss()); ?></wfw:commentRSS>
file: wp-includes\comment-functions.php line 263
PHP Code:
function comments_link( $file = '', $echo = true ) {
echo htmlentities(get_comments_link());
}
file: wp-includes\feed-functions.php line 62
PHP Code:
function permalink_single_rss($file = '') {
echo htmlentities(get_permalink());
}
file: wp-includes\template-functions-post.php line 44
PHP Code:
function get_the_guid( $id = 0 ) {
$post = &get_post($id);
return htmlentities(apply_filters('get_the_guid', $post->guid));
}