It's definitely the underscore: http://usesthis.com/ is accessible, as are all the other interviews, but http://_why.usesthis.com/ isn't.
And it doesn't seem to be the browser's fault:
$ ping richard.stallman.usesthis.com
PING richard.stallman.usesthis.com (74.207.245.100) 56(84) bytes of data.
64 bytes from waferbaby.com (74.207.245.100): icmp_seq=1 ttl=51 time=63.6 ms
$ ping _why.usesthis.com
ping: unknown host _why.usesthis.com
So I can't resolve the domain name for some reason. This happens on both my home Gentoo box and my Slicehost Gentoo server.