As Dana notes below, David Souter will apparently be stepping down at the end of the term. According to Nina Totenberg, this means that (barring tragedy) Stevens and Ginsburg will be staying on for another term. It is very unusual for a modern justice to retire while still in good health, but it is obviously less surprising coming from Souter, the old-fashioned rural eccentric who no longer lives with his mother in the family house in New Hampshire only because she moved out. (It also says something about the modern Republican Party that Souter would choose to wait until the party of the president who chose him was out of office.)
Obviously, the short-term effects of an Obama nominee replacing Souter on the Court are likely to be modest (although, as Ilya Somin notes, this is less true in the long term). Still, although Souter was certainly a part of the Court's more liberal wing, he was (as befits someone who was essentially an old-school New England Republican) a cautious, pragmatic one. Obama certainly could choose to replace him with a liberal who would be a strong liberal counterpoint to Alito, Scalia, Thomas, and Roberts. Although undoubtedly a lot of pundits will be urging Obama to preserve his "political capital" by choosing a cautious moderate (which may well be his inclination), I agree with Dahlia Lithwick that there's no time like the present for restoring the Court's liberal wing.
One good roundup of potential justices is here. Unfortunately my guess is that Obama will want to give Elena Kagan -- my preferred candidate among the most-discussed options -- some more seasoning as solicitor general and perhaps some time on the appellate bench.