If nobody knows Steve Job's email, then how can we confirm that the headers indicate it was sent by him?
We "know" nothing, however as others have said, Occam's razor basically applies here - all the other explanations are more convoluted and require more jumps in logic.
I don't see why it's hard to believe that Steve Jobs would reply to a question like this with a "No." For one thing, it sounds like him. For another thing, I doubt he'd see it as big news that there'd be no tethering. It's not like they're planning to have some big announcement at MacWorld or something that the iPad and iPhone won't tether. Oh no! Steve Jobs just spoiled his next "oh, one more thing!" No tethering!
Also, this story has actually been around for a couple of days now and nobody from Apple has denied it.
Lastly, I just want to say that email addresses are funny. Most celebrities and public figures have email addresses that would be the first thing you'd guess. I can't remember what the exact address in question here is but when I first saw the story elsewhere, I wasn't surprised that it was something like firstname.lastname@example.org. The reason people think "nobody knows" these addresses is simply that random emails from outsiders usually get ignored, and also because most people think the most obvious address would *never* actually be real. But even most celebrities, in my experience, don't really care *that* much about keeping their email address private, especially because they all have other, *actually* private email addresses that they use for important stuff, like chatting with friends or family. The professional email address just has a bunch of filters that separate all the random stuff out into junk folders.
I actually get to work with a lot of celebrities for my work, and the first few times somebody handed me their card with an address like "email@example.com" on it (that's not real - though it may be! - but just an example), I was really surprised. But now I realize how common it is.
I have no idea why Jobs would respond to this one email, but maybe he was just searching through his junk mail box and it piqued his interest for some reason.