As someone who has never been presented with a diagnosis, but clearly exhibits its symptoms, I can say a few things about ADD. Until narzos pointed it out to me, I had never heard of ADD and I have difficulty believing that it is actually a consistent syndrome with a common source. Rather it's just 'something I was born with'.

I do think it's closely related to Asperger's syndrome.

Some observations.

It wears off.
When I was 4-14 years old I would enjoy myself by doing things like counting to a thousand. I used to have a great memory for facts, which helped me in school, and gave me a reputation of being intelligent. I was fascinated by sets, arrays, collections, logical order, systems. I found it completely impossible to approach strangers, or to approach anyone without a good rational reason. I didn't know the concept of 'smalltalk' or talking about 'personal' things. I made a point of never attending any parties. I hated birthday celebrations.. I did have close friends, with whom I would play Monopoly for months on end, or something similar. I didn't socialize. These Asperger-like things gradually wore off as I got older. I suppose they are normal for children, and to grow up is to overcome them. Or maybe it's just hormones.
It doesn't mean I'm insensitive.
I am sensitive, in the same way that other mammals are, in a nonverbal, intuitive sense. What I find hard is to be sociable, and this includes sensitivity as social behaviour. I will be the closest friend you'll ever have - unless you expect me to do something for you.
It doesn't hurt.
In my daily life the worst problem I have is to organise myself. I'm always late with everything, if I show up at all. I won't, and can't, plan ahead. Given a plan I won't stick to it. I'm lazy. Somehow, other people seem to have a sense of urgency and focus; I don't. It doesn't worry me enough to turn things around. Sure I try to improve, and on occasion I do, but trying to get rid of it would be like trying to get rid of my legs: it's part of me.
It doesn't mean I'm nervous
About all this fidgeting movement. Please do tell me to sit still and concentrate - I always forget, and it really helps - but don't suspect any psychological trouble behind it. It's just the way I am.
It doesn't mean people don't like me.
Everybody likes me. That is how I get away with all the failure and the systematic underachievement; one smile, one clumsy gesture, and people will see me for the harmless fool that I am, and forgive me. Competition frightens me, judging others is beyond me - from the first day of kindergarten I tried to stay out of the pecking order. This is why everybody likes me: I'm totally harmless. If, by mistake, someone takes real interest in me and starts to expect things, I'll get them straightened out soon enough.

My true role in life is to be a clown. That's not bad. Meanwhile, I'm trying my hand at some of the things that other people find important - working, having meaningful social relationships, pretending my life has purpose, blah blah blah. I usually enjoy it; sometimes I even manage to achieve something. While some things in life are a little out of reach for me, I'm just as well off as the next person. But I can imagine how a more serious degree of the same traits can lead to serious problems.

Further observations.

Bob Seay's description at http://members.aol.com/BevKPrice/HTML/web43.html
if you ever need a guide for "things you'll notice when dealing with rp", this is it
David Keirsey's point of view at http://keirsey.com/addhoax.html
this really isn't about ADD, but rather, about the basic moral values and attitudes of psychotherapy
the various nodes on the Myers-Briggs type indicator, which seem to be altogether more useful than any single label like ADD or ADHD could ever be. I'd rather just call myself an ENTP or INTP