I agree with the others -- diet has no connection to ADHD. It would be super if that happened to be the case, but there's just no link. ADHD is a brain disorder and has to be treated as one.
I second the recommendation to watch ADD and Loving It. Terrific, factual information about ADHD and it's presented in a light way.
I also recommend subscribing to ADDitude magazine and joining CHADD to get access to reliable information about this condition. Look for message boards devoted to ADHD so you can avoid a lot of the "change the diet, remove toxins, try this vitamin" misinformation that doesn't apply to ADHD.
A couple of great books to read are "Buzz: A Year of Paying Attention" and "ADHD & Me: What I Learned from Lighting Fires at the Dinner Table."
Also, keep an open mind about medication. No parent is eager to try it and we all hesitate at first. But medication, combined with therapy, has the absolute best track record in treating this condition. Medication completely transformed our son's life. He went from being out of control and being kicked out of preschool to being pleasant and thriving at home and at school by kindergarten (however, keep in mind medication doesn't work the entire day, so when it's out of their system, you're still dealing with ADHD head-on). It can take some trial and error to find the right medication, but working with a specialist, it's worth the leap of faith to give it a try.
Best of luck to you and your family!