It turns out that the URL used includes an & that is not properly understood by some HTTP clients including pingdom and curl. To fix this, you need to edit app/code/core/Mage/Page/Block/Html/Head.php and change the 2 instances of “&,” to “&”.
I have also published the change at github so you may use that if you wish.
According the letter of the HTTP caching specification, user agents should never cache URLs with query strings. While Internet Explorer and Firefox ignore this, Opera and Safari don’t - to make sure all user agents can cache your resources, we need to keep query strings out of their URLs.