I think you're confusing Souter with Kennedy. Souter was picked by GHW Bush and Kennedy was picked by Reagan. While it's true that Kennedy sometimes does not side with the Scalia side of the court (most recently in some of the WOT ruling), he's a fairly conservative justice.
Souter, on the other hand, virtually never sides with Scalia, and has never ruled with the conservatives on any of the landmark cases that have come before him during his tenure. Despite being nominated by Bush, he has been one of the most reliable "progressive" justices in the last 25 years. His departure will do nothing to change the balance of the court.
I hadn't considered that. So as you say, if that is the case, then the court doesn't change. We may end up better off than I thought. Hopefully the only vacancies that occur for Obama are the hard left justices, meaning the court will still swing right from time to time.