Planned Parenthood v. Casey is a 1992 case that was famous for upholding Roe v. Wade, thanks to a trifecta of Republican-appointed Justices (Kennedy, Souter, and O'Connor) that joined with the Court's liberals. Thanks to Lee Epstein of Northwestern Univ. School of Law, a new digital archive of the papers of Harry Blackmun is coming online. Blackmun seems to have kept copies of all the "cert. memos" from various years on the Court -- these are the memos written by one Justice's clerk and circulated to the Court as a whole in order to recommend whether the Supreme Court ought to grant certiorari, that is, whether they should hear the case.
The cert. memo for Planned Parenthood v. Casey is now available here. It was written by Justice Clarence Thomas's clerk Stephen McAllister.