For the amount of *nix people I see showing out on here, I'm surprised that it's only 10% - although for the Web in general, that's an absurdly high number, still!

Some percentage of Windows visits could be browsing at work, maybe?

Hmm...

HN could use the user's ISP to guess whether they were home or at work, then correlate that with OS data.

That's actually an interesting idea!