First I'd like to point out "Join The Impact," which is organizing national protests, at the moment for November 15th but I expect other dates will emerge if nothing changes.
Second, I've been reading the text of the ACLU's writ petition (PDF format) which they've submitted to the Supreme Court, and I feel comfortable saying that their argument is convincing. I'd like to, for the benefit of those who are allergic to legalese, try to explain what it's about.
Turns out -- and I didn't realize this before -- there are two different ways to change the California Constitution. One is amendment, which may be done through the initiative process that was used to pass Prop. 8. The other is revision, in which the proposed change must go through the legislature before being submitted to the people for their vote. The key distinction is that an amendment adds something new that does not conflict with the current constitution, the principles underlying it, or the principles underlying the system of government. You can't reintroduce segregation by amendment (even if that weren't covered by the US Constitution as well) because it's in conflict with the principles underlying the constitution. You can't declare a dictatorship because it conflicts with the principles underlying the system of government. Etc.
Now, at the time that Prop. 8 was first being drafted, it would have been an amendment. There was no actual understanding that same-sex couples possessed the right to marry, though marriage (to the person of one's choice) has been known to be a fundamental right in California since 1948. That changed when the court ruled. Suddenly, the law officially recognized that the fundamental right of marriage belonged to gay, lesbian, and bisexual Californians too, under the principles underlying the California Constitution.
Thus, Prop. 8 is no longer a legitimate amendment. It is now officially in conflict with the principles underlying the constitution, and as such, counts as a revision. So it can't be valid law, they argue, because it wasn't passed in the appropriate manner.
A bigger problem, and one that they touched on, is this: we have three branches of government in the US, and in the state of California. The legislative branch makes law, generally only if it has the cooperation of the executive branch, and then the judicial branch makes sure that the law is consistent and just. The judicial branch's chief purpose is to ensure that the majority, through their representatives in the legislature or directly through the popular vote, doesn't abuse its power and the minority. If the majority thinks that a minority shouldn't be allowed to vote, or own land, or live in the nice part of town, or marry who they choose, it's the courts' job to prevent them from just making it so because they can.
Thus, the court acted properly in overturning the majority opinion that banned same-sex marriage the first time. They were protecting a minority from the tyranny of the majority. Now, what Prop. 8 did was basically say "no, the majority really wants this so you can't stop it." And if you think about that, that destroys the purpose of the court system completely if allowed to stand, and thus ruins minority protections. Suddenly, hey, if the majority wants to take your rights away, they'll do it. Doesn't matter if you know you're constitutionally guaranteed them -- if the majority dislikes you, they can override anything they want. It's a very dangerous thing for all minorities.
I don't think there's any question what the Supreme Court will say. The question is, will the people understand? Will they remember their education, all those years ago, about the branches of government? Or have we, as I fear is true, lost our reverence for the courts in our fear of "activist judges"? I'm afraid of backlash, when the court speaks. One way or another, this is getting uglier.