Appointed by H.W. Bush in 1990, Souter is considered a part of the liberal wing of the Supreme Court (the liberal wing being Stevens, Souter, Ginsburg and Breyer). During the presidential election it was speculated that Stevens or Ginsburg would be replaced during next presidential term. The oldest member of the court and the member to have recent cancer surgery, respectively. So, Souter's announcement is a bit of a surprise.
President Obama will likely look for a relatively young justice with a similar ideology as his own -- obviously, that's what happens. However, I doubt there will be much change in the direction of the Roberts Court.
Roberts has been careful about maintaining the independence of The Court. He speaks well John Marshall's leadership of his court, and seems to model his court similarly. (Marshall, also known as "The Great Chief Justice," was Chief Justice from 1801 to 1835.) Marshall's contributions include issuing single opinions, which sometimes meant narrowing the opinion to a very specific holding; and his earning the court the power of judicial review with Marbury v. Madison. Actions promoting the independence of The Court.
With Roberts's leadership, and because the similar ideology between Souter and any possible Obama nominee, it is unlikely there will be much change in the highest court because of Souter's decision to leave at the end of the term. It will be interesting to see how the president handles the nomination process, and who he chooses. I have very little doubt, given the current composition of Congress, he will get his wish (if the nominee makes it out of Judiciary Committee -- thanks to Arlen Specter), but quite a bit of doubt that we will see Change.