Boy you hit one of my biggest pet peeves. This is what I call "hidden disabilities" and my son has many: epilepsy, autism, hemianopsia (a specific type of partial blindness). To look at him though, he appears healthy.
I once had a man yell out his car, "That's a HANDICAPPED spot, you know!" I just stood there flabbergasted (well, I actually didn't just stand there as I was in the process of opening the door to help my son out.)
I've thought about this alot since then. I think it's better to tell people about the disability, if you're so inclined to speak about your (or your child's) disability. The reason I feel this way is because when my son was first diagnosed at three months old with having had a stroke, I had never heard of anyone other than an older adult having strokes. It felt quite surreal, and also, quite lonely. Then as I spoke with my son's therapists, I found that he was not the only infant that any of them had treated who'd had a stroke. I formed the first pediatric stroke support group in the country, so no one else would feel the uncertainty of facing this alone. (Our group is no longer in existence, but there are online groups now that are)
Education about all disabilities only helps everyone. Perhaps telling about a disability and erasing just a tiny bit of ignorance will have another positive affect down the road that you may not even forsee. I heard that after telling an acquaintance about my son, her cousin's child was exhibiting some behaviors that my son has and later -- because of the information passed along -- was subsequently diagnosed with high functioning autism and now is getting help.
I know everyone feels differently about this, and although I don't go around announcing my son's disabilities, when appropriate I am very upfront about them, and my son is too. To our family, his disabilities are a part of who he is, just as the fact that he has brown eyes is a part of who he is. Just since I was a child I've seen a huge shift toward accepting people of all abilities, race, sex and religion. The way DS is treated in school is completely the opposite to the way some kids with obvious disabilities were treated when I was in school. Often the unfamiliar is feared, but with knowledge comes acceptance.
I'll get off my soapbox now. Thank you for your kind consideration of another opinion.