There is a basic FAQ is on this public site and they have a link to more information.
http://www.atf.treas.gov/pub/nfab/index.htm
There is a FAQ at this commercial site (sorry, it is a frames site so you have to select the How to Own file). On their FAQ page they also have a link to a compilation of which states allow autos:
http://www.vectorarms.com/indexframe.html