There's still nothing stopping you from emailing him a question after that kind of exchange on a blog, unless he doesn't post his email address publicly.
I totally agree (I'm reasonably comfortable with total strangers emailing me), but if I, say, meet someone at a trade show then there seems like there's a higher level of acceptability to it than "hey, remember me, I responded to your blog post." Maybe it's just me, though?
And I have to disagree with this... or at least, I don't want the distinction to become arbitrary. The Internet and Web were meant to make communications easier. But it seems like all this online social network crap is replacing real world meet-ups.
