Speculation now centers on whom the President will nominate to replace him. Here's a prediction: Whoever it is will have the intellectual heft that Sonia Sotomayor conspicuously lacked. With his first nomination, President Obama secured a predictable left-wing vote and courted the Latino community (after he and his fellow Democrats treated the eminently qualified Miguel Estrada shamefully). But I suspect that Obama longs to find a lefty jurist equipped to match the rapier intellect of Antonin Scalia (good luck!) -- and someone in whom he sees reflected his own self-image as an "intellectual."
Of course, the value of a sharp left-wing jurist is evident in the career of Justice William Brennan. Even when Brennan wrote dissents, he tried to provide an intellectual framework for the reinterpretation of American law, with an eye to moving it to the left. And often, he succeeded. That's a success Obama would love to replicate.