My mother has it. She has tried just about everything BUT diet, so I couldn't tell you how diet has or has not helped her. But in The China Study, Dr. Campbell suggests that there appears to be an emerging connection between dairy and all auto-immune diseases, including arthritis, fibromyalgia, psoriasis, lupus, and others. The mechanism is not clear but thought to be that casein (the main protein in milk) is capable of mimicking vitamin D in the body, thereby acting as a Vitamin D antagonist.
I think he may also have cited some studies that appear to show that many arthritis sufferers get worse when they have dairy and better when they don't. Arthritis is not fibromyaglia, of course, but it's in the same family of diseases. (My mother hates milk and has been lactose-intolerant for decades.)