In the episode airing 11/5/08, "Rumble", the issue of Jack McCoy's effectiveness as District Attorney was questioned as one of the episode's themes. At the very end of the episode during a press conference, you hear a reporter shout a question, obviously directed at McCoy, if he has been asked to join the Obama administration. That shouted question appears to have been edited in since the original airing of this episode was only the day after the Presidential election. Since McCoy's character has been the District Attorney his role has be lessened in series. If Waterston is leaving the show, he's has the opportunity now. However, I cannot locate any press release.