I think the trouble is now, if this was just released, is that you can easily google for c99.php and find vulnerable servers with this file hosted on it, from pre-existing backdoors/phishing hosts, or what have you, and this allows anyone and everyone to use it, not just the original person who placed the backdoor. Which, I suppose, kind of enhances the problem exponentially.

As far as I'm concerned though, Google and Firefox's malware checker engines should blacklist any domain that has the c99.php file located on it and block their webbrowsers from connecting to it in the first place.

Of course - correct me if I'm wrong here.