From my reading of the case, I think we're most likely to see SCOTUS either just deny standing, or make a narrow ruling. I very much doubt they'll uphold Prop 8, but it's also unlikely that they'll make a broad ruling.
Of course, if they make a narrow ruling, then I think the logical approach for gay marriage advocates is to sue somewhere else, preferably in a state that never granted gay marriage in the first place. Although seeing their waffling, I'm no longer quite sure that they would make a nationwide ruling given a lack of other options.