Steve,
I agree that it is not law until they successfully defend it in court as they did in the "once a machinegun" issue. Then again, they may lose it like they did the Thompson-Center buttstock issue. Until then, people take a risk that BATF will enforce the law as interpreted by the letter ruling. They, the defendants, could prevail in court, but that is a potentially expensive defense.
As for the complete weapon question, until you attach a functional, semiautomatic upper receiver to the lower receiver, or have all the parts to one in your possession, the receiver fails to meet the definition of a [red][b]semiautomatic rifle[/b][/red] capable of accepting a detachable magazine, and is therefore not a 921(a)(30) assault weapon and not subject to exemption under 922(v)(2) regardless of the original data of manufacture/assembly. Of course, that doesn't apply to a named assault weapon.
BATF's interpretation of the law appears to be a narrow, and very literal one. A successful arguement of the "Once an assault weapon, always an assault weapon" theory would appear to require the arguement that legislative intent was that the 922(v)(2) exemption be permanent and not rely on the weapon remaining in 921(a)(30) configuration to retain exempt status. Just how important is legislative intent?
(Edited because my fingers are typing "x" instead of "s".)