Unfortunately even though the feed validated I received the following warning:
wfw:commentRSS should be wfw:commentRss
The explanation given on the site was:
Due to a clerical error, the element Chris Sells originally spec’ed as wfw:commentRss appeared as wfw:commentRSS (note the difference in case) on Joe Gregorio’s site. At least one product (WordPress) has implemented and deployed feeds using the spelling originally present on Joe’s site.
And the solution:
Change wfw:commentRSS to wfw:commentRss
Wordpress users can find this in the wp-rss2.php file.
Gotta love nice and easy fixes :)