Man there is a lot of misinformation in this thread. I will break this down in some decent detail, but it is late, I am not documenting jack **** right now, nor am I looking things up so this is going from memory. However, this is a topic that has interested me and I have studied the cases in some detail. Take what I say for what it is worth.
First, there are two very different cases before SCOTUS in terms of SSM. The first is Perry v Hollingsworth. This is where marriage as a right will be looked at, maybe. This is also where many people are getting confused. The facts: marriage in this country is a right. Most frequently cited to prove that is Loving v Virginia, but there is a whole plethora of cases that do the same thing. You may not like it, but until SCOTUS rules that it is not a right(and think hard about that, do you really want that to be the case), or the constitution is amended, marriage is a right.
To deny some one from a right requires the state to be able to do certain things, depending on the right and who is being denied that right. One of the big questions in this case is the level of scrutiny that should be used. Even at the lowest level of scrutiny, to deny SSM couples from marrying, the state would have to show that the law is rationally related to a legitimate government interest. In other words, the state would have to show that prop 8 serves a legitimate government interest. The question would be what is the government's interest in limiting marriage to only being between a man and woman. That is the lowest possible hurdle, and it could be much higher depending on how the court rules.
However, it is quite likely the court will sidestep the issue altogether. Easiest and most likely would be a ruling that denies the petitioners(prop 8 backers) appellate standing. basically this means that the people who appealed can't. Even that get's complicated, but the most likely outcome would be going all the way back to Judge Walker's decision, which would likely still be put on hold. Temporary stalemate. Next most likely, ruling that since SSM couples did at one point have the right to marry, the state cannot take it away. Then there is the possibility the court could decide that the petition was granted in error, meaning that in reality the SCOTUS should not have heard the case at all, and thus confirming the appeals court ruling.
There are a number of other possible outcomes. Best guess, 50 - 50 that SSM will be legal in California as a result of SCOTUS ruling. If not, then most likely it will be held in legal limbo as lower courts have to go through the process again. It is very unlikely that the court will simply uphold prop 8, but certainly not impossible.
Note that nowhere in there did I mention equal rights. There is a reason for that. While equal protection is a part of the case, simply saying it is an equal rights issue is vastly too simplified. Also note that those who are claiming that marriage is a right are, well, wrong. It is a right. Further note that the definition of marriage varies by state currently, and has varied widely in the history of our country, so toss out any of those traditional definition arguments, they fail.
The other case is Windsor v United States, and no that does not mean we are fighting Canada. This is a much more clear cut case which has a much more likely outcome. States have historically been the ones to define who can get married in their state, and other states and the federal government recognize those marriages. DOMA pretty clearly takes away the right of the state to do that. This is an argument which appeals both to conservative and liberal judges. This is highly likely to be the outcome of the case(probably 80 % likely). The big question is whether the court will rule on portability(that is, whether if a SSM couple marries in a state that allows such marriages moves to a state without, does the federal government still have to recognize the marriage?) and a few other technical aspects. Most likely not, which would leave the decision basically in Obama and the justice department's hands, at least for now. Once again, it is however possible that the court will decide not to rule, and my basic understanding is if that happens, DOMA would last awhile longer until another case makes it through the system or it would put it back to the lower courts ruling and DOMA goes away.
So to recap: most likely DOMA is gone, but this is far from a sure thing. Prop 8 has so many possible outcomes, who the **** knows how that will turn out.