"We're told the Galaxy Nexus-compatible build will be available for download within the next few weeks"

No serious company wants the responsibility of bricking someone's smartphone.

Where it would get interesting is if they introduced this as a UML guest Android app.

It would be an impossible task to do that for all phones. But the community can make ROM's for different phones like they do it for Android.

Canonical must have at least one ROM that they use for their own testing. Why won't they even release that?

Information around the web suggests that it's being tested on the Galaxy Nexus, and that the image for that will be released.