So I installed Virtual Box 2.10 in Windows XP and suddenly the host adapter is gone, like it never even existed. This really foils my instructions.
Now, though, I can use any network adapter as a host adapter.
So what's a boy to do? First, set up a Microsoft Loopback Adapter. Then follow my instructions but everywherre that it says "host networking interface on the host computer" replace that with the Loopback Adapter. It works!
So now I am communicating between 192.168.1.1 on the host loopback and 192.168.1.2 on the guest. Nice, although I'm not happy that Sun changed this without telling me. Or maybe they did somewhere, but I looked at the docs and couldn't find anything.
Edit: After a long an unhappy session at a cafe with WiFi, I realized that it's not okay to just make up addresses that a real router might be using. Finally I've settled on 192.168.5.1 and 192.168.5.2 (guest). Now I'll configure my Ubuntu in a VirtualBox...