FEDERAL LAW ON TRANSPORTATION OF FIREARMS
or local laws which would prohibit the passage of persons with firearms in interstate travel.
Notwithstanding any state or local law, a person shall be entitled to transport a firearm from any place where he may lawfully possess and transport such firearm to any other place where he may lawfully possess and transport such firearm if the firearm is unloaded and in the trunk. In vehicles without a trunk, the unloaded firearm shall be in a locked container other then the glove compartment or console. Necessary stops, e.g., gasoline and rest, seem permissible.
from this page
http://www.nraila.org/GunLaws/FederalGunLaws.aspx?ID=59