I've heard of this diet. I got a flyer for it from the local health foods store and asked my mother if she'd heard of it, since she works with a lot of dyslexic and ADD/ADHD kids. She said she had heard of it and (I think) that some of her clients had tried it. I shouldn't put words in her mouth, but my impression was that she wasn't too impressed but thought it couldn't hurt, either. I've also talked to some other folks who find they really react strongly to foods, so there may be a population this diet might really help.

Also, I'm not anything close to an expert on ADD/ADHD, but my impression is that sometimes overly intense concentration can be part of it as well. I wouldn't dismiss concerns of it just because concentration is sometimes possible. (On the other hand, I wouldn't necessarily advocate drugs for it, though they, too, can really help some people. My parents keep pushing me to get checked and try them since my father has ADHD and he's convinced I must have ADD, too. He may be right, but don't tell him I said so!)