While I am not a graduate legal scholar yet - I will be going to law school in a couple of years - I have a been a keen lay-follower of our Supreme Court since I was 12 years old.
While I have read many articles in the mainstream media that seem to suggest that President Obama will end up nominating a slightly left-leaning pragmatist moderate, I am not convinced of this. I don't believe that Mr. Obama will appoint an outspoken liberal, either (i.e. a liberal Scalia).
I feel that President Obama will nominate a highly-intellectual pragmatist liberal-libertarian nominee in the mold of Justice William Brennan. Justice Brennan was an extremely intelligent liberal-libertarian thinker who sought to convince his more moderate or conservative colleagues to see his way of thinking and, more times than not, end up voting his way. Justice Brennan did not accomplish his persuasive goals through bravado, insult, or manipulation, but rather through cogent and rational argument.
President Obama is intelligent enough to realize that you should not pick a nominee based upon considerations of race, ethnicity, or gender. Decisions regarding who one will appoint for lifetime terms on our judiciary (regardless of level) requires far-deeper levels of consideration. If a woman or an African American or a Mexican-American ends up getting the nod for Souter's seat - fine. I just don't think that these superficial traits and classifications should be a primary consideration.
My favorite candidate for Souter's seat is Professor Lawrence Tribe, although he is probably too old to stay on the Court long enough to have an appreciable effect on the tenor of the Court's operations. My two top choices after Professor Tribe would be either Kathleen Sullivan, a Stanford Law professor, or Russ Feingold, a U.S. Senator from Wisconsin.