Perplexed, I just posted a topic in Parenting that asks some of the same questions you bring up here. I also have a sensory seeker and to me it seems like his physical "hyperactivity" is related to sensory overload in a sense, rather than the inability to pay attention. We have also had a few issues in school with him "not paying attention" although it hasn't been complained about too much. When I asked him about it, he said that he understood what the teacher was saying the first time she said it and he didn't see why he needed to listen to her repeat it several more times! So I also think that kids who pick up on things quickly may sometimes appear to be inattentive, but like your DS, are perfectly capable of participating even when they don't appear to be paying attention.
I also think there are expectations about what someone who is paying attention looks like: sitting still, quiet, eyes on the speaker. But one can assume this position without really paying attention OR, especially for sensory seekers, one could be paying attention without being still or looking directly at the speaker. And then how frustrating to be scolded for not paying attention!
