SCOTUSblog weighs in:
Trying to sort this all out, it is clear that the Court has agreed to consider the merits case in Prop. 8, because that is what the petition presented as its question, but that it is also going to address whether the proponents had a right to pursue their case. If the Court were to find that the proponents did not have Art. III standing, that is the end of the matter: there would be no review on the merits of Proposition 8, or of the 9th CA decision striking it down.
UPDATE: The decisions have been posted.