Same sex marriage was legal in CA before Prop 8 repealed it.
That's actually the heart of the legal decision: in a state where same-sex marriage has been approved, it can not be taken away by another group. So the Supreme Court will not make a decision on the right to same sex marriage. What they will do is make a decision on the right of one group to take away the rights of another group after those rights have been granted recognized.
This is helpful for states like Washington and other states where same sex marriage is recognized but currently threatened. But it does not change things for states that have already and only banned same sex marriage.
It is absolutely a good and necessary step, but even a ruling on Prop 8 by the Supreme Court will not be the end of this issue.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack