Update (Mar 6, 2009): Following the arguments heard in the California Supreme Court on Mar 5, immediate media reports say that the justices are inclined to uphold the law although the court did not vote or issue a ruling. The decision is due within 90 days. According to the LA Times, only two of the seven justices "indicated a possible readiness to overturn the initiative."

Los Angeles Times: California Supreme Court signals mixed response to Proposition 8:

The California Supreme Court appeared ready today to uphold Proposition 8, the November ballot measure that banned gay marriage, but also seemed likely to decide -- perhaps unanimously -- that the marriages of same-sex couples who wed before the election would remain valid.

During a three-hour televised hearing in San Francisco, only Justices Carlos R. Moreno and Kathryn Mickle Werdegar suggested that the court could overturn the marriage ban as an illegal constitutional revision.

Time.com: Gay Marriage: Is California's Supreme Court Shifting?:
The prospects of same-sex marriage in California grew dimmer Thursday, when two Supreme Court justices who helped create the right for gays to marry in last year's historic decision expressed deep reservations about attempts to strike down a statewide referendum passed last fall to ban the practice. "You would have us choose between these two rights: the inalienable right to marry and the right of the people to change their constitution," said Justice Joyce L. Kennard, one of those two key judges. "You ask us to willy-nilly disregard the right of the people to change the constitution of the state of California. But all political power is inherent in the people of California."

San Francisco Chronicle: Justices seem to be leaning in favor of Prop. 8:
The California Supreme Court, which last year declared the right of gays and lesbians to marry, appeared ready Thursday to uphold the voters' decision to overrule the court and restore the state's ban on same-sex marriage.

"There have been initiatives that have taken away rights from minorities by majority vote" and have been upheld by the courts, said Chief Justice Ronald George. "Isn't that the system we have to live with?"

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With some 18,000 legally married same-sex couples in California and millions of observers anticipating the outcome, the California Supreme Court will hear arguments seeking to overturn Proposition 8 on the grounds that such a constitutional change requires approval by the state Legislature on Thursday.

Last November, 52 percent of California voters approved Proposition 8 which reversed the May 2008 state Supreme Court decision to legalise same-sex marriages. The measure amended the state Constitution to declare that marriage only between a man and a woman is valid or recognised in California.

Immediately after the November election, two groups of same-sex couples and local governments led by the city of San Francisco filed lawsuits challenging Proposition 8. The lawsuits argued that a measure depriving a minority of fundamental rights is such a drastic change to the state constitution that it is a revision, which exceeds the power of initiatives. Opponents of the Prop 8 also argued that the measure violates the constitutional separation of powers by preventing the judiciary from protecting a minority group.

California Attorney General and former governor Jerry Brown has encouraged the legal challenge. Brown, who initially said his office would protect the measure, later declared that he would be unable to argue in favour of it. He says Proposition 8 is unconstitutional because the Supreme Court's 4-3 decision last year recognised gays as a minority group entitled to judicial protection and established marriage as a fundamental right. Gov. Arnold Schwarzenegger also opposed Proposition 8.

Media reports say the court is also expected to decide the validity of same-sex marriages that were sanctioned in the state before Proposition 8 passed if the measure is upheld. Some 18,000 same-sex couples were legally married from May to November last year.

According to the Los Angeles Times, most legal experts expect the court to uphold current marriages but also to keep Proposition 8.

The decision is due within 90 days.

The court will hear oral arguments on Thursday, March 5, 2009, from 9:00am (GMT -8 hours) to noon. The arguments will be telecast on the California Channel and will be shown publicly on a JumboTron in San Francisco's Civic Center Plaza and in several other locations. A live Webcast will be available at www.calchannel.com.