While I'm generally a little too concerned with what other people think about me and my family, when it comes to my 5 year old's ADHD, I don't give a hoot. I wouldn't be ashamed or concerned about telling people if he was diabetic, and, frankly, it helps explain a lot of the behavior that is smacking them in the face. He's a total riot and people love him, but he's exhausting to watch.
Not everyone "gets" it, and when they don't and are vocal about it, I typically knock them down a peg or two by given them a little more truth about ADHD. In my son's case, an MRI has shown a frontal lobe issue, so when I get the old, "You just need to keep him more busy and find better ways to discipiline him," I simply tell them that they need to be very careful about commenting on things they know nothing about, as they run the risk of being offensive.
I only tell when I get to know the parent and they notice something amiss then I share. In most cases it brings out understanding. I told almost everyone. Most people who know are friends and familyI've made no secret of my son's dx- the whole family knows, as do most of our neighbors and about 1/2 pf his friends parents. I've never gone out of my way to tell someone about it (other than family), but when something pertaining to it comes up I don't hesitate to share. The only people I've gotten a negetive reaction from are family.
I'm also a firm believe in combating the misperceptions surrounding mental health issues and the only way to do that is by educating people. The fact that schizophrenia was considered the fault of the mother until not that long ago shows how people's perceptions can change with a little info and some science to back it up.
Our 5 yr old son was dx in early Nov, we told grandparents, my sister & her husband, school and two of our closest friends (only because they have children that play w/him and I felt it was helpful for them to know). I have otherwise been reserved to talk about it much because I feel that it is something that not many understand. Until you live with it, it's impossible to fully understand everything that goes along w/it. We just felt that those that deal w/him often or daily needed to be aware of what is underlying in his bad behavior or inability to concentrate. Also, we are medicating & still trying to find a "good fit" so those same people also need to be on the look out for changes in him, good or bad.
I also have a mother like yours, who thinks he's "just a busy boy" "he's too smart for his own good & isn't challenged enough" "he'll 'outgrow' these impulses and naughtly behavior". As much as I try to educate her on what we are learning, she still only half-heartedly accepts what we are truly dealing with. One minute she's asking questions about what we've learned and seems to be trying to understand, but then the next minute she has the "been there, done that" answer because she's raised two kids. 
Our son is "wicked bright" also, which tends to make it somewhat harder to understand why he does the things he does, if he knows better, but I'm constantly reminding myself that is what ADHD is all about--even though they know, they can't control themselves. I do also wonder if he will struggle even more in school because he may NOT be challenged enough, which will add to the struggle to stay focused if he's bored.
This is a great website for advice, information and just looking for someone who has "been there". We feel like we are still "new to this" also and learning things everyday. Good luck to you and your family for finding helpful ways of dealing with your challenges! 
Hello all! I'm a newbie in many, many ways - I just stumbled onto your site today (thanks to a boss out of town! time to surf at work!)
I'm also new, in terms of seeing my daughter's behavior in light of ADHD & anxiety (I'm sure my story is not that unusual - she's in 1st grade - has always been different, pre-school and school have been mixed bags, good when good but horrid when bad. Nobody could put a finger on it, the H having been virtually invisible in toddler years. She's wicked bright, too - which is a blessing and a curse. Finally seeing a school-recommended child psychologist, who I love, and have opted for now to not dx or med but just trying behavior mod, with some improvements.)
I found myself searching out this site mostly for companionship. I have mom friends, each with their own joys and hurdles, who've all been supportive but who mostly (except one with an ADHD son) don't really understand.
I'm curious how many people in your lives know about your kids' condition. Surely many know something is different about your kid - but all kids are different. Do you tell your kid's friend's parents - what about after a really bad incident? What about the doting gramma who insists nothing is wrong with her little Einstein grandchild that your being a better parent wouldn't fix?
I admit I've decided not to tell my parents yet. Maybe my dad, who knows more about our struggles - but my mom, forget it! She's an MH counselor and will debate DSM-IV characteristics with me if I try.
Welcome! Glad you found us here. I've told neighbors, relatives, and family friends who have seen some horrible behavior from my child. No matter what judgments they make about ADHD, they can't be worse than what they were thinking about my child after witnessing his hyperactivity and impulsivity! My decision to tell these people has worked out well. I also tell all of his teachers: classroom teacher, reading teacher, PE teacher, etc. The reason to tell them is to help them know the best way to work with him and so I can have close communication with them about how he's doing, just in case things come up that need addressing.
I'll add that I gave a different answer to the moms who tried to get my child kicked out of his transitional kindergarten program (this was pre-diagnosis, so this was pre-meds). I don't fault them for being concerned that their child would get injured from my child's pushing, impulsively tripping people, etc. However, my "gut" told me that these ladies would not have a good attitude about ADHD. So, here's how I handled it. My ds was diagnosed over the summer and responded really well to meds. The problematic behaviors just disappeared! So, when I ran into one of those moms at the open house prior to school starting, I told her -- knowing that she would tell all those other moms -- "I thought you'd like to know that this summer my child was diagnosed with a medical condition. He's now on medication to control it, and all of those problem behaviors have disappeared because they were a side effect of his medical condition." She sounded sincerely happy for me, and I could tell she wanted to breathe a sigh of relief that my child no longer would be a menace and a danger to others. She asked me what kind of medical condition he had. I told her that I didn't want to go into details about it. She pressed again, but I just repeated that I didn't want to go into details about his medical situation. She then dropped it. I did the same thing when the school's secretary said to me in a kind but curious way "The first few months of school have been going so great for your child. What did you do to change things?" (He'd been in the principal's office a lot prior to starting ADHD meds).