From the Boston Globe:

O’Connor will hear arguments from lawyers for both sides — the State of New Hampshire and the reproductive rights group Planned Parenthood — but may not be on the court when the case is decided. If she has been replaced by the time the justices decide the case, her vote will not count. If the court is tied, 4 to 4, the case would be reargued next term, and O’Connor’s replacement could be the deciding vote.