Justice David Souter intends to retire at the end of the current court term, NPR reports.  Souter was apparently confident that neither Ruth Bader Ginsburg or John Paul Stevens will retire at the end of the term.

Nina Totenberg reports, 

Given his first appointment to the high court, most observers
expect Obama will appoint a woman, since the court currently has only
one female justice and Obama was elected with strong support from
women. But an Obama pick would be unlikely to change the ideological
makeup of the court.

Though appointed by President Bush, Souter became part of the liberal bloc on the Court, and he voted to uphold the core holding of Roe in Casey vs. Planned Parenthood. Totenberg mentions "Elena Kagan, the current solicitor general who represents the
government before the Supreme Court; Sonia Sotomayor, a Hispanic judge
on the U.S. Court of Appeals for the Second Circuit; and Diane Wood, a
federal judge in Chicago" as possible successors.