It seems the IWF have backed down in their attempt to censor Wikipedia. One of the reasons they cite is the unintended consequence more people having seen the questionable image since they pointed it out. This is sometimes referred to as the Streisand effect after Barbra Streisand's attempt to censor the internet.
There are a number of issues raised by the whole affair including questions about censoring stuff on the belief it may be against the law. However I'd like to concentrate on the technical issues involved.
The Cleanfeed system is actually fairly well thought out. For starters they only filter URLs rather than whole domains. This means an offending page can be filtered out without knocking out a whole site. I'm sure people can imagine the outcry blocking the whole of Wikipedia would have caused.
They way the filtering is handled is by passing your web-request through a transparent proxy. Obviously if everybody had their web surfing routed through this one proxy everyone would suffer from slower surfing. For this reason Cleanfeed utilises a two stage process where only requests to sites in the IWF blacklist are forwarded to the proxy. Unfortunately for UK Wikipedia users this meant that all requests to the site went through the proxy. From Wikipedia's point of view this meant the entire UK population where making requests from a single IP address. As an IP address is the unique identifier of a device connected to the internet Wikipedia use it as a way of blocking sustained vandalism. The side effect being that all anonymous edits of Wikipedia where blocked once some-one had committed some vandalism through the proxy. Another unintended consequence of the IWF filter.
There are a number of lessons that can be drawn by the whole episode. However this case is should be held up as a example to those people that think technical solutions to filtering the internet are workable. Unintended consequences indeed.