Sahil Kapur of TPM outlines the possibilities. There are some twists, e.g., lower courts unexpectedly granted the right to appeal the law to the originators of Prop. 8... not the California government, which declined to defend it... and the Supremes may decide they didn't have standing. So the article is worth a read.
I'll add only one observation. We say casually that the Supreme Court "decides" the constitutionality of a law. But at least two Supreme Court members, Scalia and Thomas (who almost always votes with Scalia), and possibly more, have already made up their minds, and Scalia is even talking about it. If that isn't questionable judicial ethics, I don't know what is... broadcasting one's pre-judgment of a case before even hearing it. But The Fat Over-aged Catholic Choirboy will push the limits, and apparently no one sees fit to try to stop him. So I am confident there are two votes on the Court to reinstate Prop. 8 before proceedings even begin.
This is the most politicized Supreme Court ever, and it was created that way by a succession of Republican presidents nominating Justices well right-of-center who lied their way through confirmation hearings to gain seats on the bench. If, over a long enough time period, Democrats nominate judicial moderates and Republicans nominate off-the-wall right-wing nut-jobs, the bench will tilt so far to the right that the building's stability will be endangered. That is where we stand today. No president ever nominates new Justices as far to the left as virtually all Republican nominees are to the right. So for a while we're stuck with a Court of wing-nuts appointed to serve an agenda.
Let me say for the record that ruling Prop. 8 constitutional would be an injustice on a massive scale affecting our most personal of rights: the right to formalize legal recognition of our most intimate partnerships. Tens of thousands of gay people have already gotten married in California alone: stating now that the law grants none of the legal benefits of marriage to gay couples legally married is reneging on a fundamental, once presumed irrevocable, promise. If we cannot depend on the government to keep its bargains, what can we depend on?