It doesn't matter.
I mean it matters, but it doesn't matter. Souter was a liberal justice (despite being appointed by George H.W. Bush) and the way the Court is currently constructed his leaving probably won't change anything. Obama will appoint another liberal justice who may or may not be further to the left of John Paul Stevens.
So while the media writes stories in the past tense about Souter, his retirement does not change much of anything. Unless Obama's pick to replace Souter is to the right of Justice Kennedy (and let's face it, the Cubs have a better shot at winning the World Series), the Supreme Court will continue to look much like it does now. Only the names behind one of the votes will change.
Right now, the most 'important' member of the Court is Justice Kennedy who is currently the median voter. He is the swing vote. Justices Stevens, Ginsberg, Souter, and Breyer are liberals. Roberts, Alito, Scalia, and Thomas are conservatives. In a controversial case, things pretty much always work out that way, it is 4 to 4 and Justice Kennedy comes in and casts the deciding vote. Souter leaving doesn't change the math.
So what really matters on the Court is when/if Scalia or Kennedy decides to retire (they are 73 and 72 respectively). If one (or both) retire when Obama is President, the the nature of the court changes.
The most recent shift in the Court was back in 2005 when Justice O'Connor retired. She had been the swing vote. When Alito/Roberts replaced her (remember Cheif Justice Rehnquist died after O'Connor announced her retirement) the Court did take a step to the right as Kennedy then became the swing vote. Here is an old (for blogs) post on that shows that and touches on how the court was going to change after O'Connor's retirement.
[I should note that being the median voter on the Supreme Court makes you one of the more important and powerful persons in the United States. While, Kennedy probably leans to the right, he votes with the liberal justices enough to prevent total conservative domination on the Court.]
While it's fun to write really crappy articles/posts about who will replace Souter, the fact is, it doesn't REALLY matter. It's a fun Beltway story to talk about over a gin-fizz. Whomever replaces Souter will most likely be young, much like Roberts and Alito (and Thomas), to give the Court a Baby Boomer liberal voice. He/she will probably be to the left of Breyer and maybe Stevens and Ginsberg, (which then puts Breyer in position to be the median voter if Kennedy or Scalia retire). However, other than that, don't expect the Court to change all that much in the short term.