My guess?
Ginsburg, Kennedy, Souter.
http://www.washingtonpost.com/wp-dyn/content/story/2008/03/25/ST2008032502998.html
Joining Roberts were the justices who are most consistently conservative: Antonin Scalia, Anthony M. Kennedy, Clarence Thomas and Samuel A. Alito Jr.
Justice John Paul Stevens concurred, but for different reasons than Roberts gave. Stevens agreed that Texas could not be forced to reconsider the case but urged it to do so nonetheless, especially because its failure to advise Medell¿n of his rights “ensnared the United States in the current controversy.”
Justice Stephen G. Breyer wrote in dissent that the court had misread the supremacy clause of the Constitution, which says properly ratified treaties “shall be the supreme law of the land” and that the treaties at issue did not need to be implemented by congressional legislation. “As a result, the nation may well break its word even though the president seeks to live up to that word and Congress has done nothing to suggest the contrary,” Breyer wrote. He was joined by Justices Ruth Bader Ginsburg and David H. Souter.