In the article he was critical of the British Chiropractic Association (BCA). Specifically, he questioned their claims that manipulation of the spine can cure illnesses in children such as asthma, ear infections and colic, by altering the flow of blood around the body (these claims bear a striking resemblance to those made by proponents of Braingym). Its difficult to find a transcript of the original article, since many websites are scared of being sued for libel along with Dr. Singh. However, I did find a copy on a Russian blog, here.
The result of his article was a two year legal battle (costing him 200 000 pounds), after the BCA sued him. He has consistently defended himself on the basis that he had the right to "fair comment". In an initial hearing, the judge found in favour of the BCA, but this decision was reversed in the High Court a few days ago (1st April, 2010).
The most obvious result of the whole thing has been the mobilisation of science writers and bloggers in the UK in support of Singh, and the highlighting of inequities within UK libel laws. Many people (including celebrities, journalists and politicians) are currently calling for reform of the law because of their perception that it favours large organisations with the money to silence their critics. Dr. Singh has always claimed that science writers in particular must be allowed to publish without fear of being sued by the organisations they are investigating.
I wasn't previously aware of British libel law, but it seems that it has gained some notoriety over the years. The UN's Committee on Human Rights has attacked "libel tourism", where foreign businessmen and millionaires use the High Court in London to sue foreign publishers under defamation laws. Similarly, in one episode of South Park, Tom Cruise shouts at the American press "I'm gonna sue you...... in England."
Simon Singh's website can be found here, together with his petition to reform the law (which I will admit to signing myself).