Today, the California Supreme Court unanimously held that Prop 8's supporters have standing to defend it on appeal, and can argue its merits before the Ninth Circuit when the Governor and Attorney General have declined to do so.
Because this question of whether California's initiative law created standing for an initiative proponents was a novel one under California law, the Ninth Circuit (a federal appellate court) had certified it for consideration by the California Supreme Court, which could speak to this question authoritatively, and agreed to accept its decision as binding (as California's court rules require). Moreover, the parties to the litigation themselves agreed that if standing existed under California law, it exists under federal law as well.
So why did the Court so rule? This section from the introduction states it pretty clearly:
[B]ecause the initiative process is specifically intended to enable the people to amend the state Constitution or to enact statutes when current government officials have declined to adopt (and often have publicly opposed) the measure in question, the voters who have successfully adopted an initiative measure may reasonably harbor a legitimate concern that the public officials who ordinarily defend a challenged state law in court may not, in the case of an initiative measure, always undertake such a defense with vigor or with the objectives and interests of those voters paramount in mind.As loathsome as Prop 8 itself is, one can too-easily imagine a counterexample which a Republican administration would refuse to defend a liberal initiative in court, such as a hypothetical effort to overturn Prop 209 and instead require California's universities to employ affirmative action practices in admissions, or an initiative imposing new limits on campaign financing.Allowing official proponents to assert the state‘s interest in the validity of the initiative measure in such litigation (along with any public officials who may also be defending the measure) (1) assures voters who supported the measure and enacted it into law that any residual hostility or indifference of current public officials to the substance of the initiative measure will not prevent a full and robust defense of the measure to be mounted in court on the people‘s behalf, and (2) ensures a court faced with the responsibility of reviewing and resolving a legal challenge to an initiative measure that it is aware of and addresses the full range of legal arguments that reasonably may be proffered in the measure‘s defense. In this manner, the official proponents‘ general ability to appear and defend the state‘s interest in the validity of the initiative measure and to appeal a lower court judgment invalidating the measure serves to enhance both the fairness of the judicial process and the appearance of fairness of that process.
...Neither the Governor, the Attorney General, nor any other executive or legislative official has the authority to veto or invalidate an initiative measure that has been approved by the voters. It would exalt form over substance to interpret California law in a manner that would permit these public officials to indirectly achieve such a result by denying the official initiative proponents the authority to step in to assert the state‘s interest in the validity of the measure or to appeal a lower court judgment invalidating the measure when those public officials decline to assert that interest or to appeal an adverse judgment.
What this means in substance is that the constitutionality of Prop 8—an initiative which took away marriage equality from California spouses after it had already been recognized by the California Supreme Court—will be ruled upon by the Ninth Circuit and, potentially, the United States Supreme Court thereafter. The Ninth Circuit has received all the merits briefs and held oral argument on December 6, 2010, so it may issue its ruling at any time.