The case is Perry v. Schwarzenegger. You can watch live on C-SPAN. I'll update with comments soon.
UPDATE: You can read some details about the case here. I'll have some of my own impressions in a little while.
UPDATE 2: You can watch the whole oral argument here. The first hour of the argument dealt with the threshold question of standing. California Governor Schwarzenegger and Attorney General Jerry Brown were the original defendants in the case, and they have opted not to appeal. Proponents of Prop 8 (which banned same-sex marriage) need a way to intervene using some other party with a personal stake in the outcome or the decision of the district judge, finding Prop 8 unconstitutional, will be the final word on the subject.
It seems as though there should be appellate review, but the constitutional limits on federal court jurisdiction don't depend on how important it is for an issue to be heard. There are technical requirements, and it seems as though the plaintiffs' lawyers did a clever job of setting things up so a district court decision in their favor would be insulated from appellate review. But listening to the oral argument, I got the impression the judges thought it was too clever.
In the second hour, the substantive question boiled down to whether there was a rational basis for excluding gay people from marriage. The pro-Prop 8 side rested heavily on the fact that only heterosexual couples produce babies accidentally. But that has so little to do with the value of excluding gay people from the status of marriage. It's hard even to understand why reserving marriage to heterosexuals would make them do a better job of deploying their reproductive powers. Why should gay people, who aren't even the problem, bear the burden?