You've outlined it really well — the DNS system seems to be series of servers that we've all agreed to trust. It would be easy enough to set up your own servers and direct popular domains to the wrong place or have a gateway to some alternate Internet. I don't know how those much talked about "darknets" work, but I know enough to imagine some techniques they might use. The idea of there being a secret second Internet makes me feel like a kid looking for a secret passage behind the bookcase, but alas if there really is a secret Internet then it's probably very seedy. And thanks for the Jon Postel story! I love these "man behind the curtain" kinds of Internet yarns. Vint Cerf's memorial for Postel was very entertaining and moving at the same time.
When IPV6 becomes widespread I predict we're going to happen to see a lot of new "private network" spaces which will be large, wide spread networks which use the public ipv6 space, but which use acls to allow specific networks to access them. We do this a bit now with NAT, but ipv6 could do this on huge scales and still not exhaust the available ips.The idea of there being a secret second Internet makes me feel like a kid looking for a secret passage behind the bookcase, but alas if there really is a secret Internet then it's probably very seedy.