The Supreme Court has just held that the Second Amendment, interpreted in Heller to protect a natural right to self-defense by ensuring access to firearms, applies to (i.e., limits) the states as well. This probably (but not necessarily) means that Chicago's gun ordinance, which has the practical effect of preventing private citizens from owning guns, is dead.
My off the cuff reactions:
1. The privileges and immunities clause, which for more than 100 years has been thought to be dead, is really, really dead.
2. Justice Thomas' lone opinion that the Second Amendment applies against the states because of the privileges and immunities clause, rather than through the due process clause, was necessary for him to concur in the result because he disbelieves in due process clause incorporation (as do I).
3. Thomas' opinion is defensible, because the right to keep and bear arms really is a privilege of federal citizenship and arguably the amendment applies against the states on its own terms ("the right of the people" (emphasis added)), although the Supreme Court has long since rejected that idea.
4. This opinion will be a seven day wonder, much like Heller and the corporate free speech decisions, with the media lamenting conservative judicial activism. A few laws will be tweaked and someone in Chicago will get a new gun, but not much will actually change.
5. The only exception might be that McDonald will weaken the NRA. Not much existential threat to gun ownership if it's a recognized constitutional right.