Chris at The Smoke Filled Room argues that it is possible the next president will not have that big of an impact on the Supreme Court's long-term future.
Based on the relative ages of active senators, Chris notes that Justice Stevens could try to outlast a second Bush administration. In that case, Bush would get to appoint at most two justices, and would be replacing two conservative-leaning seats in Rehnquist and O'Connor, so the overall ideaology wouldn't shift all that much. On this last point, I tend to disagree, since it is likely that Bush would appoint an O'Connor replacement with less of a tendency to vote with the liberal wing on certain issues.
If Kerry is elected, Chris argues, he would probably only get to replace Stevens, thus only trading one liberal for another, with the Chief and Sandy holding on a little longer (until the Guliani administration?).
All of this is simply a fun game to keep us all busy. We have no idea what will really happen. After all, any number of things can happen in four years. Thomas could retire to drive around the country in his r.v., Ginsburg could become a professional poker player, Souter could get mugged again, etc.
Only time will tell.