Is there an official build planned though? Or is Google only planning on supporting "official" hardware?
ChromiumOS, (build or download built) on anything it runs on
You can't build the browser Chrome yourself, because that is Google's trademark. But you can build Chromium.
There is little difference between ChromeOS and ChromiumOS, so it doesn't matter. What's wrong with something built outside google? Chromium will have all the beta features first, so it'll be better.
I guess the question really comes down to whether or not Google is planning significant work on the hardware hooks into the OS - will it be possible for manufacturers to write ChromeOS drivers for their devices, or will Google develop that on their own?