Prediction

Based on what I heard and read yesterday (including at the SCOTUS blog, among other places), my guess is that the Sup Ct is going to say it should never have taken the Calif. marriage case (certiorari improvidently granted, to use the technical lingo) and thus leave standing the 9th Circuit ruling which invalidated Prop 8 on fairly narrow grounds. Which would count as a win for the pro-marriage-equality side but would have no implications outside of California.

Update: J. Sides links to an amicus brief of twelve political scientists in the Calif. case.