...let's assume that Justice Souter's replacement always votes exactly as Souter himself would have. So long as the rest of the Court remains the same as today, nothing will change. However, if Obama is then able to replace even one of the five more conservative justices, the balance of power would become very different than it would have been had Souter been replaced by a more conservative justice than himself. What would have been a 5-4 conservative majority will become a 5-4 liberal one. Justice Antonin Scalia, for example, is 73 and could eventually be replaced by a liberal Obama appointee - especially if Obama wins reelection in 2012. Moreover, Souter's replacement will likely serve for decades to come. So Scalia's possible replacement by an Obama appointee is just one of many events that could happen during the tenure of Souter's successor that could make his or her ideology extremely important.
In theory, if Scalia is replaced by a liberal in 2012 and Souter himself remains on the Court, the ideological balance will be exactly the same as if Souter were replaced by a younger ideological clone of himself in 2009. However, the "younger" part is a key distinction between the two. Souter's replacement is likely to be much younger than he is, and will therefore probably be around a lot longer than Souter would have been had he chosen not to retire this year. Thus, he or (more likely) she will be affecting the Court's ideological balance for many years longer than Souter would have been able to do.
For these reasons, Souter's replacement will matter a great deal even if he or she has little immediate impact on the ideological balance on the Court.