From Deadline|London editor Tim Adler: You may be upset or relieved to hear my news that Howard Stern definitely won’t be joining American Idol as the judge replacing Simon Cowell. According to my sources close to the show, the new judge will be announced at the end of the current run. Whoever gets the gig will have a sweet deal: 19 Entertainment, the London-based company which created American Idol, has a budget of $38 million to spend on Cowell’s replacement.

http://www.deadline.com/hollywood/

It was obvious that Stern wouldn't do it. There's no way he would agree to the censorship.