The Heller decision, published yesterday, ruled that the District of Columbia cannot impose restrictions on guns that would completely ban weapons for self-defense. The scope of the case is limited to Washington DC however, and it is an open question as to whether other extremely restrictive anti-gun measures will be affected or not. The reason for this is the incorporation doctrine of the 14th Amendment to the U.S. Constitution.
The Bill of Rights, when written, did not apply to the states. There must be some federal question or federal jurisdiction in play before the Bill of Rights will be invoked. But, the Supreme Court has also held that certain parts of the Bill of Rights are so important that they must be applied to the states as well. The court does this via the due process clause of the 14th Amendment, which states:
. . . nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
Many parts of the Bill of Rights have been incorporated and now apply to the states as well, such as freedom of speech and freedom of religion. The Second Amendment, alas, has not yet been incorporated, and Heller did not address this issue. Nor should it have - Washington DC is obviously not a state, so the incorporation doctrine was irrelevant.
So the bottom line is, the Second Amendment must be incorporated via the 14th Amendment to apply against the states before cities like Chicago and San Francisco will have their very restrictive anti-gun laws overturned.
Another questions is just what restrictions would still be allowed after Heller. Scalia, writing the majority opinion, went out of his way to say that restriction on gun ownership and use are valid, but did not get into specifics. No amendment to the Bill of Rights is without restrictions. Not even the vaunted First Amendment is sacrosanct, and the Supreme Court has recognized 'time, place and manner' restrictions on the exercise of free speech, for example.
The state constitutions have their own versions of the Second Amendment, and Illinois has a great example with Scalia's restrictions built right into it:
SECTION 22. RIGHT TO ARMS
Subject only to the police power, the right of the individual citizen to keep and bear arms shall not be infringed.
'Police power' here does not mean the cops - it is a legal term of art that means "the capacity of a state to regulate behaviours and enforce order within its territory, often framed in terms of public welfare, security, morality, and safety." Chicago has one of the most restrictive anti-gun laws in the nation, thus proving that Illinois does not take its own Constitution seriously. Completely eliminating the right to keep and bear handguns goes way beyond reasonable police power. But, the amendment perfectly illustrates what Scalia is talking about.
Thus the Heller decision opened as many questions as it answered. But I like the direction the discourse is taking!