He wrote in article in the UK newspaper The Guardian about this, and the British Chiropractic Association, unsurprisingly, took some exception to it. In fact, they sued him for libel. That lawsuit has been going on for some time, and you can find details by searching Google.
What's interesting is that Singh could have simply argued that what he said was a free speech comment (or the UK equivalent), which is what's usually done in these kinds of cases. That lets the paper off the hook, and everyone is happy... though it could be argued there is something of a chilling effect for journalists, who might be afraid to be sued again. But it's an easy solution that gets the job done.
However, it doesn't look like that's what Singh is doing-- he and his legal support have turned the tables on the BCA, and are saying that in fact what Singh wrote is true: a lot of chiropractic practices are bogus pseudoscientific quackery. This is beautiful, as it forces the BCA to defend what it does in a court of law. In other words, this goes from a simple libel case to one where an entire sector of garbage "alternative medicine" has to prove what it does is actual medicine.
The details to me aren't clear; in America I have little doubt that a group like the BCA could bring forward three witnesses who say their cold symptoms went away just three days after having their spines adjusted, and a jury would award the chiropractors a bazillion dollars. But the UK has different laws, and I suspect if Singh is taking this strategic route, he'll be well-prepared for such a tactic.
The entire skeptical community will be watching this case very closely, I assure you. This could potentially be huge, bigger than the Dover creationism trial. I would so dearly love to see medical quackery get the justice it oh-so deserves. Most pseudoscience has subtle ramifications, but alternative medicine quackery hurts and even kills people.
If you are on Facebook, then please join the group For Simon Singh and Free Speech - Against the BCA Libel Claim to show your support and also to keep up with the latest news on this very important case for reality.