I've worked at a few places where it was against the rules to talk about your salary. I asked the owner why (quoting what Spolksy had said) and I was told that

I've worked at a few places where it was against the rules to talk about your salary. I asked the owner why (quoting what Spolksy had said) and I was told that if the employees know what each other earned, half of them would leave. I asked why some people were being paid more than others for the same job, and the owner told me, "Some people are better at negotiating than others." And I knew for a fact that I am terrible at asking for more, so it was time to look for another job. Shame really. It was a good job, but I'd always feel like someone had negotiated better than me and was earning an extra 20%