The Supreme Court today held that the Second Amendment -- as recently redefined in D.C. v. Heller , in which the Court overturned D.C.'s handgun ban -- applies to the states, not just the federal government. Heller made this decision inevitable, so the only suspense with today's ruling was with respect to 1) the size of the majority and 2) what rationale the justices would use to "incorporate" the Second Amendment. The Court -- with the same 5-to-4 lineup as Heller -- chose to incorporate the Second Amendment through the due-process clause of the 14th Amendment rather than through the "privileges and immunities" clause.
To briefly explain what's at stake here: The Supreme Court essentially eviscerated the recently enacted 14th Amendment in the 1873 Slaughterhouse Cases, and in particular gave the "privileges and immunities" clause an implausibly narrow meaning. (As one of the dissenters noted, if the 14th Amendment meant what a bare majority of the Court claimed, "it was a vain and idle enactment, which accomplished nothing and most unnecessarily excited Congress and the people on its passage.") And while the due-process clause has been revived in the 20th century -- first as a vehicle for now-discredited property-rights opinions, and then as a means of incorporating most of the Bill of Rights and protecting unenumerated rights in cases such as Roe v. Wade and Lawrence v. Texas -- the privileges and immunities clause has remained an empty shell, protecting only a narrow set of pre-Civil-War rights such as the right to travel.