It's in his (Mitnick's) autobiography Ghost in the Wires. In his telling of the story they put him in a more restrictive environment exactly because of the reason given (launching nukes by whistling into a phone)
The part I can't understand is believing that phone-whistles are connected to our nuclear launch system and not simultaneously trying to put nuclear missile operators in jail for incompetence.
"Oh the world might end with some whistling? Definitely punish the whistlers, that's where the problem is..."