10:17 a.m. Getting the big news out of the way: Today's decisions do not legalize OR outlaw gay marriage nationwide. The Roberts Court has issued fairly narrow rulings that deal only with the cases at hand.
First up is the Defense of Marriage Act. The court ruled, in a 5-4 decision authored by Kennedy, that DOMA violates the Fifth Amendment's promise of "equal protection under the law." There also appears to be a defense of states' rights to define marriage recurring throughout the opinion. Kennedy says, "DOMA singles out a class of persons deemed by a State entitled to recognition and protection to enhance their own liberty." In other words, the law is designed with the purpose of giving a certain group of people superior legal tratment. It is unconstitutional.
The upshot? Edith Windsor, the plaintiff in the case, doesn't have to pay the hundreds of thousands of dollars in extra taxes the IRS says she owes. And all federal marriage benefits, from tax law to immigration to hospital-visitation rights for federal employees, are now in effect in the states that have legalized gay marriage. In the states that have banned gay marriage, gay marriage stays...banned.
Scalia read his dissent from the bench in the DOMA case, and it was a fiery one. Per SCOTUSBlog, "The Court's opinion both in explaining its jurisdiction and its decision 'both spring from the same diseased root: an exalted notion of the role of this court in American democratic society.'" Josh Barro of Business Insider has more on Scalia's blistering dissent here.
A throwaway line in Justice Roberts' DOMA dissent hints that the court will decline to rule on Prop 8 because the petitioners lack standing. More on that shortly.
Update, 10:37 a.m.
As they hinted at in the DOMA dissent, the court has decided that it cannot rule on Proposition 8 due to the petitioners' lack of standing.
What does this mean exactly? This high up in the courts, there are no longer plaintiffs and defendants — there are petitioners (the people who bring the case) and respondents (the people who are being sued). Petitioners, in other words, are the people who lost the last round and have appealed the case to the next level. Respondents won the last round.
The court has ruled that the petitioners in Prop 8, a gaggle of Republican lawyers, did not have standing to challenge Prop 8 because they could not demonstrate they were tangibly harmed by the law. If the State of California had decided to defend Prop 8, the situation would have been very different, because a state defending its own duly passed law clearly has standing. But since neither the state or federal government was willing to defend Prop 8, the task basically fell to a bunch of random dudes who just...don't like gay marriage that much.
Prop 8 has never been upheld by any court. By throwing the case out based on lack of standing, the Supreme Court has basically decided that if California won't defend its own law, no one can. This means that the decision of the previous court to take the case — that Prop 8 is unconstitutional — stands as the final verdict. Gay marriage will be legal in California.
What about the dissenters in the Prop 8 case? The court decided the case 5-4, with Roberts, Scalia, Breyer, Kagan, and Ginsberg on the winning side and Sotomayor, Kennedy, Thomas, and Alito losing. Obviously, this is not an ideological breakdown. These four justices dissented, basically, because they felt that the petitioners (i.e. the conservative lawyers defending Prop 8) did have standing to challenge the law. They claim this is the case because of California's ballot-initiative system, which in their words is designed "to establish a lawmaking process that does not depend on state officials." In California, citizens can change the state constitution by simple majority vote. The whole point of this system is that people can make their own laws WITHOUT the intervention of state lawmakers or governors. The dissenters go on to state, "This purpose [ballot initiatives, popular government, etc] is undermined if the very officials the initiative process seeks to circumvent are the only parties who can defend an enacted initiative when it is challenged in a legal proceeding."
Unclear whether this logic would still hold in a state that doesn't have a ballot-initiative system. But the dissenters' logic basically is that in a system where every man is a king, anyone has the standing to challenge a law.