The President has nominated Federal Appeals Court judge Sonia Sotomayor to replace retiring David Souter on the Supreme Court. As John McCain reminded an audience recently, "elections have consequences."
One of the consequences of electing Barack Obama President is the nomination of judges selected from the various interest groups which comprise the Democratic Party. To expect anything else is folly. Sotomayor was selected to appeal to three interest groups: diabetics, women and Hispanics.
This president received a majority of the votes cast in 2008 and is entitled to nominate whom he chooses. The Senate is entitled to confirm or not confirm those nominations and, inasmuch as there is a Democratic majority in the Senate, his nominations are likely to be approved.
Replacing liberal Souter with liberal Sotomayor is not likely to be consequential. Only when a liberal replaces a conservative, or vice versa, is the appointment likely to be important. The court's conservatives aren't particularly old and are unlikely to retire during an Obama administration. The court's liberals are older and Obama will probably get to replace many of them with younger judges.
The court's even balance of liberals and conservatives will probably continue into the forseeable future, regardless of who is president.