Oral arguments are sometimes not very good indicators of how justices will rule. In this case, I would be surprised if the oral arguments didn't give exactly an exact indication of the way the case will go, at least on the question of whether the Second Amendment protects individual rights.
Justice Scalia basically came out and said that he'd made up his mind before the argument even began. He behaved more like an advocate than a judge.
Justice Thomas, as is his practice, said nothing. Chances are that (in spite of concerns expressed by Justice Stevens) he will go with Justice Scalia.
Chief Justice Roberts and Justice Kennedy expressed plain preferences for the view that the Second Amendment created an individual right, and the only real question was whether the regulations infringed that right.
Going the other way, Justice Stevens seemed to have decided that the purpose of the Second Amendment was not to create an individual right.
Justices Souter, Ginsburg, Breyer and Alito were less transparent. Justice Alito, however, directed his questions only at the advocates arguing to uphold the ban, indicating that his vote is the crucial fifth vote on the constitutional question. (Justice Alito, more than any other justice, focused upon the interplay of constitutional provisions pertaining to the militia. Look for Justice Alito to write the majority opinion.) Justices Souter, Ginsburg and Breyer, even if they join in the result, are unlikely to join the other five justices in their rationale.
And what is the rationale? Well, this could be interesting. Justice Stevens clearly has a problem with the other justices' apparent lack of respect for the intent of the Framers. It is often said that the intent of the Framers is to be taken into account when interpreting constitutional provisions. One of the justices who supposedly champions the ability to "read the Framers' minds" and pronounce what they intended is Justice Scalia.
In the case of the Second Amendment, Justice Scalia and the others may be departing from this principle that they otherwise claim to think is important. Historically, the Framers' intent seems to be pretty clear. They were not concerned with individual rights. They were concerned with military matters. The historical record is very strong on this point. Furthermore, the notion of protecting an individual's right of self-defense was specifically proposed for inclusion, but the proposal rejected. How on Earth could the Court say that the amendment protects an individual's right to self-defense when the Framers considered and rejected that very concept?
If the remarks of Justice Scalia and Chief Justice Roberts are any indication, there will be an effort to impute to the Framers an intent that has scant overt historical support. Basically, the words of the amendment itself (which are the best indicator of the Framers' intent) do not restrict the rights to military matters. Had the Framers wanted to restrict the rights to the military context, they could have easily done so, but they didn't. Instead, the Framers may have felt that the right of self-defense was a non-controversial pre-existing right (having been endorsed by the great William Blackstone), and that the best way to ensure a robust militia is to make sure that there is guaranteed individual liberty to keep and bear arms. To be on the safe side, the guarantee is much broader than might have been necessary. But this broad guarantee makes sense, because tyrants generally do not take actions against militias directly; rather, they vitiate militias by keeping the citizens unarmed.
The key questions then, will be the (1) extent of the individual right, (2) the extent to which the right can be regulated by the government, and (3) the standards by which the court will determine whether a government regulation is valid. These are the real sticking points in this whole case, and these are the questions upon which unintended consequences may be quite serious. Further, the indicators from oral argument about how the justices will handle these issues is less than clear.
One possibility is to say that individual Second Amendment rights are just like other individual rights. If there is governmental action affecting them, the government better have a really damn good reason for doing so. This is formally called "strict scrutiny," and represents the highest level of justification the government must show. Speaking generally (and colloquially), courts apply the following standards, depending upon the circumstances and the rights involved: (1) a low level of scrutiny, in which the government must show that its actions are supported by a good reason, (2) an intermediate level of scrutiny, in which the government must show that its actions are supported by a really good reason, and (3) strict scrutiny, in which the government must show that its actions are supported by a really damn good reason. Under the really damn good reason standard, it's not clear that the District of Columbia has made a showing of a really damn good reason for its regulations.
But the consequences of such a ruling may be extensive. There are a lot of weapons regulations, both state and federal, and if each has to be independently supported by a really damn good reason, then these regulations may be in a shambles. This is why the Solicitor General urged "intermediate scrutiny."