Does anyone really believe anymore that S.J. himself writes all the letters / messages which are so popular lately? I sometimes don't have time to respond to everyone, even though I'm just a developer. I know many managers who could spend the whole day writing and answering emails and they're not even that high in the hierarchy. I can't even imagine how many emails must Steve get from inside the company -- and these are the emails he has to actually handle himself.
I remember an article about Bill Gates a couple of years ago saying that there was a team of ~10 people handling his mail, but can't google it now :/ I don't believe S.J. would answer himself, if the question can be handled by any assistant with access to some internal knowledge-base. (just like this one)
Edit: not what I saw originally, but: http://money.cnn.com/2006/03/30/news/newsmakers/gates_howiwo... <- in 2006 B.G. got only ~100 emails a day from people worth his attention - the rest got filtered out / handled by assistant. And there was already some time after Vista. S.J. probably gets more right now since the iPad release.