Dahlia Lithwick (with whom I rarely agree and for whom I rarely oft provide a link) ponders that very thing in an excellent article.
My only issue is that he thinks Obama will appoint a "like-minded liberal centrist" to fill the next spot. Now I agree that Obama will certainly find someone who is like-minded (especially to Ginsburg--the only Democrat-appointed member of the liberal wing who may be retiring soon), but not that the person will be a 'liberal centrist'. Lithwick would contend that there is no Scalia of the left--the progeny of a William Brennan or Thurgood Marshall--but that doesn't mean the liberals on the Court are centrists. The difference between Brennan and Ginsburg is that Ginsburg doesn't make a big production out of it when she shreds the Constitution to fit her politics.
Anyway, it's worth the read.