Solicitor General Elena Kagan has been nominated for the vacancy created by Justice Steven's retirement from the bench. Her nomination is almost sure to be confirmed. Democrat's don't have a filibuster proof majority in the U.S. Senate, but they don't need many Republicans to determine that this nomination is not so "extraordinary" that it requires a filibuster. Maine's two moderate Republicans (both women) and several other moderates from the GOP are likely to prevent a filibuster from being effective.
The Democratic base is also unlikely to engage in an uprising that causes the nomination to be withdrawn, as the Republican base did once during George W. Bush's administration. Kagan has already been confirmed by the Senate to an important senior post and is a known quantity who has been considered a front runner for the post for months now.
Sometime this summer, or perhaps early this fall, she will be confirmed and take her seat as the most junior justice on the highest court.
This won't change the balance of power within the Supreme Court, of course. She will be a reasonably liberal replacement for a liberal justice. Justice Kennedy will remain the swing vote on the court. But, this appointment makes it much less likely that a later Republican President from entrenching a conservative supermajority due to the ill health of a liberal justice or two. With his second Supreme Court appointment, President Obama, acutely aware of the issues involved in his appointments as a former professor of constitutional law, will have made his mark on the court.
Commentators so far have focused on how the ability of a future Justice Kagan to bring conservative Justices to her point of view may, however, change the nation's jurisprudence.
These two appointments, made in the first two years of Obama's term, also leave open the possibility that a justice's ill health or another resignation would afford him one or two appointments to the Supreme Court.