I've been thinking about asperger syndrome as I've always been inbound and never acting out. Besides I've got a cousin diagnosed with quite severe asperger. But then again, a lot of people are mentioning ADD here.
My little boy (adopted) seems to be showing signs of autism, which wouldn't be surprising since his dad suffers from it himself. ADHD, however, was mentioned as an alternative and his school said that could make more sense, so it's easy to see crossover here!
I think it's good to explore all possibilities. It's good to be educated about these things - especially since misdiagnosis seems more common in psychiatry (anecdotal).