The saga continues...
Someone keeps attempting searches like source:http://img35.pixiv.net/img/shikishima_e/21274264* which unfortunately is fairly slow. It's spread across multiple IP addresses which makes me think this isn't some isolated service just leeching the site.
For this reason, post XML/JSON feeds are now restricted to members only. An easy way is to just pass in login and password_hash params along with your GET request.
Provided you are authenticated, I've removed the request throttle, although please still limit your request rate.