Supreme Court justice David Souter is rumored to be planning his retirement starting in the next judicial session (which kicks off in October). That means Obama will have the chance to make his first Supreme Court appointment.
Souter, though appointed by a Republican President - George HW Bush - has evolved into one of the more reliably liberal voices on the bench. Therefore his retirement is unlikely to tip the balance of the judiciary, since Obama will presumably appoint a fellow liberal to replace him. But it does give the President an opportunity to shape the bench - a task which, as a former Consititutional Law professor, he should be well qualified for.