I seem to recall hearing arguments to the effect that Prop 8 needs to go uncontested for exactly this reason - when the Supreme Court rules on something, it's pretty damn hard to get them to change that ruling, and so there's a theory that the best way to ensure high-level legality of gay marriage is to wait a few years until Obama's appointed a few new judges, and then challenge something to bring before the court.
We may not have a liberal majority on the Supreme Court during the Obama Administration.
Ages of the current Justices:
Stevens: 88
Ginsberg: 75
Scalia: 72
Kennedy: 72
Breyer: 70
Souter: 69
Thomas: 60
Alito: 58
Roberts: 53
Rehnquist died at 80, O'Connor retired at 75. As for other justices who've retired in the past couple of decades, Blackmun was 85, White was 76, Marshall was a few days shy of his 83rd birthday, Brennan was 84, and Powell was 79.
Lots of justices stick around until after they're 80 -- while it's possible that Kennedy or Scalia may retire before that age, it's not certain by any means. It's very unlikely that either of them will retire during Obama's first term, and it's pretty damn early to be talking about whether he'll have a second.
And barring something very unexpected, Thomas and Alito won't be leaving during Obama's administration OR his successor's, and Roberts later still.
Obama's certainly going to be nominating Stevens's replacement, and probably Ginsberg's, but there's no guarantee anyone else's seat will open during his administration. Kennedy and Scalia may stick it out longer than Souter or Breyer simply for strategic reasons -- let's not kid ourselves and say Stevens is still on the court for any reason besides waiting until Bush is out of office.