In an article in Slate about restrictive diets, Daniel Engber noted that
Celiac patients have almost twice the normal risk of cancer, and one-third of them suffer from another autoimmune disease, like Type 1 diabetes, lupus, or multiple sclerosis.
Does celiac disease cause cancer, Type 1 diabetes, lupus, and multiple sclerosis? Not very plausible. Does cancer cause celiac disease? Does lupus cause celiac disease? Not very plausible. Much more plausible is that all five have a common cause. I believe that common cause is a malfunctioning immune system due to not enough bacteria in the diet (the umami hypothesis).
More (May 2012). I now think that all these diseases are due to wheat molecules leaking into the blood and setting off an immune reaction that attacks parts of the body (because the wheat molecule resembles those molecules). The leaky gut that allows wheat molecules to enter the blood is caused by lack of bacteria in the diet.