Morgenthau on Guns
Robert Morgenthau is highly critical of the Thune amendment, which would allow people with concealed-carry gun licenses to carry their guns across state lines. According to Morgenthau, this is a violation of the Tenth Amendment.
I wonder if similar reasoning can be applied to marriage licenses (so if a state banned gay marriage, gay marriages performed in other states would be null and void in that state), drivers licenses, or high-school diplomas. We might have a situation in which diplomas from a state with a Creationist school system are not recognized elsewhere or vice versa. (Remember that vice versa. Political power can work both ways.)