Escape hatch, thy name is standing.
Standing - an issue that was part of the Ninth Circuit's analysis in the Perry case and thus fair game in the Supreme Court - is a fairly obscure aspect of constitutional law. It basically means that the federal courts will adjudicate disputes only when there is a genuine "case or controversy," ie, when all parties have something genuinely at stake, other than their opinions.
When the constitutionality of a state law is in question, state officials defend it...except when they don't. The governor and attorney general of California both declined to defend Prop 8, and thus the question arose of whether the proponents of the initiative, allowed to intervene in the District Court, had standing to appeal to the Ninth Circuit after District Court Judge Vaughn Walker ruled Prop 8 unconstitutional. The California Supreme Court said the proponents would have standing in the state courts, and the Ninth Circuit accepted that interpretation for purposes of the Perry case, holding that the state (as distinct from state officials) can delegate standing, thus empowering Prop 8 proponents.
But federal courts can have different rules for standing than state courts do. Which means that the Supreme Court could rule that the Prop 8 proponents lacked standing to appeal to the Ninth Circuit, even if they would have had standing in state court.
Bottom line: What will be the result if the Court decides the case based on this ground, without reaching the merits of whether the Constitution forbids states from denying gay couples access to marriage? Happy day.
If the Court of Appeals lacked proper jurisdiction because the proponents lacked standing to appeal, the Ninth Circuit decision will be vacated. Then the question becomes whether the District Court had proper jurisdiction. Because state officials were defendants at the trial level and because intervention by the proponents was permissible (although arguably not required), the Walker opinion should stand. There are multiple reasons why its precedential value will be minimal: it's the opinion of one trial court judge, not binding on any other court; and because the case was not brought as a class action, it technically will apply only to the individual plaintiffs and a couple of county clerks.
Yes, but - The politics of the issue will effectively resolve the legal questions. Regardless of whether they have to, officials in California will treat the Walker ruling as binding. Technically, the court clerks in conservative counties might refuse to go along, which could require new litigation. But polls tell us that the majority of Californians no longer would support Prop 8, and so its eventual death is inevitable, more for political than for legal reasons.
In the process, Jerry Brown may emerge as the true hero of this case: as state attorney general at the time of trial, he filed a brief declaring Prop 8 to be unconstitutional, and as the current governor, he's certain to accede to the Walker ruling (as is the current AG Kamala Harris). Credit too for former Governor Schwarzenegger, who also declined to defend Prop 8.
So, back to the Supreme Court - what will it do? Predicting results in Supreme Court cases is usually a fool's errand. This case, especially, could produce a fractured outcome, with some opinions turning on the standing question and others on the merits. It may be that no single opinion will command a majority of the Court. But the insertion of the standing question in the grant of review means that some Justices may be inclined to rule on that basis. And that might well make the difference between victory and defeat.