The oral argument will be three hours long, from 9 a.m. to 12 p.m. You can watch it at this site. If in the state you can also watch the oral argument on the California Channel, a cable public-affairs station. At its website, the court has a page devoted to the Prop 8 challenge, including links to all of the briefs filed on either side.
The main argument is that Prop 8 was a "revision" (not an "amendment") to the state constitution requiring prior legislative approval and not simply a majority vote of the people. The state attorney general, Jerry Brown, makes the different and, shall we say, more creative, argument that same-sex marriage is among the "inalienable" rights protected by the state constitution that simply cannot be taken away. Ken Starr has been making the primary case for upholding Prop 8 as a valid "amendment" requiring only a simple majority vote.
On the revision vs. amendment question, the text and history of the initiative process created in 1911 tell us little. The initiative process was a Progressive reform designed to circumvent a state legislature thought to be dominated by wealthy business interests. Whatever else can be said of them, the contending sides in the SSM debate are not robber barons.
The state court precedents don't provide a clear answer, either. As usual, both sides can point to decisions (and to isolated passages in decisions) that support their positions. One side argues that California has a long history of direct democracy, which the state courts have generally honored by liberally allowing constitutional changes as "amendments." The other argues that in light of last May's marriage decision, In re Marriage Cases, Prop 8 is unprecedented in form: it strips a fundamental right from a vulnerable minority. Starr's briefs in the case are lively and excellent in many ways, but they never really come to grips with the California marriage decision and its potentially game-changing significance for the Prop 8 challenge. That doesn't mean Prop 8 will lose. I've previously written that I think the petitioners have a good argument, but I still doubt it will be a winning one.
The other important question raised by the case is the status of the 18,000 same-sex marriages entered between June and November 2008. The plain text of Prop 8 supports an argument that these marriages are invalid, in my view ("Only marriage between a man and a woman is valid or recognized in California.") So the main issue is whether an explicit statement about retroactive effect must be included when legislation or constitutional change produces an important alteration in a citizen's status and rights.
Tomorrow, throughout California and across the country, there will be "viewing parties" to watch lawyers and judges thrust and parry over Prop 8. It's the Super Bowl for SSM, though we won't know the winner for weeks (the court has 90 days to issue an opinion). It'll be the final word, until the next final word.