(Looks like the reason Bill Ayers wasn't convicted was the fault of Mark Felt, of Deep Throat Watergate fame, because of his illegal actions)
In Letter to 'NYT,' Man Who Prosecuted Weather Underground Hits Linking Ayers to Obama
By Greg Mitchell
Published: October 10, 2008 9:00 AM ET
NEW YORK In a surprising a letter to the editor published in The New York Times today, the chief prosecutor of the Weather Underground in the 1970s expressed outrage over the linking of Barack Obama to Bill Ayers by the McCain campaign, adding, "Although I dearly wanted to obtain convictions against all the Weathermen, including Bill Ayers, I am very pleased to learn that he has become a responsible citizen."
William C. Ibershof also corrects a charge in the Times: "I do take issue with the statement in your news article that the Weathermen indictment was dismissed because of 'prosecutorial misconduct.' It was dismissed because of illegal activities, including wiretaps, break-ins and mail interceptions, initiated by John N. Mitchell, attorney general at that time, and W. Mark Felt, an F.B.I. assistant director."
Felt, of course (you may have already forgotten), was also known as a guy called "Deep Throat."
(Full letter at link)
http://www.editorandpublisher.com/eandp/news/article_display.jsp?vnu_content_id=1003873017