Souter was appointed by former President George H.W. Bush, but often voted along liberal lines. Therefore, it is unlikely that Obama's appointment will substantially alter the ideological makeup of the Supreme Court, although a more liberal member could have some effect.
Souter dissented in the controversial case, Bush v. Gore, which ultimately stopped the couting of votes in the State of Florida, thus handing the Presidency to George W. Bush. Vice President Al Gore had won the popular vote. Additionally, In Planned Parenthood v. Casey, Souter wrote that the landmark decision in Roe v. Wade, affirming abortion as a fundamental privacy interest, should not be overtuned because doing so would be do bend to "political pressure."
It is thought that President Obama will have at least two appointments to the court, although most speculation centers on Ruth Bader Ginsburg and John Paul Stevens due to health issues and age, respectively.