There are 9 members of the US Supreme Court (Pop Quiz: How many Can You Name?) but really the two that everyone will have their eyes (and ears) on are Anthony Kennedy and John Roberts. Kennedy has written the two most pro-gay decisions in the court's history (1996's Romer v. Evans striking down Colorado's Amendment 2 and 2003's Lawrence v. Texas striking down the country's remaining sodomy laws) so he is expected to play a major role in determining how the court will rule in Hollingsworth. Roberts is the Chief Justice, and as he showed with last year's surprising decision to uphold the Affordable Care Act (i.e. Obamacare) he is a consummate politician who is always trying to maintain the legitimacy of the judicial branch in our system of government. Roberts has also caused headlines because his openly lesbian cousin will be attending the oral arguments in person as a result of action by the Justice. He is the head of the conservative wing of the Court, but is thought to be pragmatic enough to not want the two biggest gay rights cases of his generation to be 5-4 decisions.
We'll know in June 2013!