Some people think this may offer the court an "out" to avoid finding one way or the other on marriage. If the Prop8 supporters lack standing (since they can't demonstrate any actual harm to them caused by Prop8), the whole thing becomes rather messy, but remains limited to California.
From Linda Greenhouse at the NY TImes:
Standing has been an issue in the Proposition 8 case ever since the state of California decided not to appeal Federal District Judge Vaughn Walker’s 2010 ruling that the proposition was unconstitutional. The appeal to the United States Court of Appeals for the Ninth Circuit was carried on by a group of people who had worked to get the proposition adopted. The Ninth Circuit questioned whether this group had the requisite Article III standing, and asked the California Supreme Court to tell it whether under California law, a ballot measure’s proponents are regarded as properly standing in the state’s shoes if the state decides not to defend the measure. When the state court answered yes, the Ninth Circuit took the answer as sufficient and proceeded to decide the appeal, finding Proposition 8 unconstitutional.
Whether standing under state law translates into standing for the purposes of Article III is a question that the United States Supreme Court has poked at but never resolved. Whether this is the right case in which to do so remains to be seen, but it was not particularly surprising for the court to raise the issue. In fact, in an era of direct democracy run amok, with voters being presented with extreme propositions that no rational state government would wish to embrace, a Supreme Court decision on who can carry the ball into federal court is probably overdue.
This was part of the appellate litigation for Prop8, so the existing attorneys have already briefed on this.