http://www.mothering.com/articles/growing_child/food/feingold.html (Please note that I think some of the cited books and articles are pure unadultrated bullshit.)
In LeftyKid's case, he becomes easily overstimulated (mostly by crowds and noise- taking him into a big box store used to be pure hell) and acts out, and when he was younger diet was definitely a trigger for that. It seems to be less of a factor now, I don't know if it's something that he's growing out of physically of if he's just better at self-regulating his behavior and coping with the overstimulation.
My Mom gave me one of Dr Weil's cookbooks a few years back (I don't know why she buys me omnivorous cookbooks, but anyhow) and he suggests some criteria for evaluating a prepared food that I think are useful for people looking into Feingold or a similar dietarty approach (I'm paraphrasing.) What he says to ask yourself while looking over the ingredients is if they reflect how you'd prepare the food at home. So if you are looking at at package of oatmeal cookies, the ingredients should look like what you'd use to make a batch at home, and if there's corn syrup or arteficial vanilla flavor or something you wouldn't use you put it back on the shelf and look for something else. He also suggests being skeptical of anything your grandparents didn't cook with, though he uses a lot of international ingredients my Grandma certainly never heard of, so I'd revise that to mean healthful and minimally processed ingredients with a long track record.