A fascinating Ninth Circuit opinion upholding a conviction of a would-be child molester (U.S. v. Mayer), a conviction that flowed from an FBI agent's infiltration of the North American Man/Boy Love Association.
The Ninth Circuit rejects, in my view quite rightly, the First and Fourth Amendment challenges to the conviction. One of the ways that the government can fight the harm caused by groups that advocate illegal conduct, whether NAMBLA, the KKK, the Communist Party, or what have you, is precisely by watching the group closely, including from the inside. The groups ought to have the right to speak; but the government is entitled to listen, at least from any vantage that any private citizen can listen from.
I should note, incidentally, that this is not simply a "the government is watching our association" case. The agent not only infiltrated NAMBLA, but also (1) "published an article in the [NAMBLA] Bulletin and wrote a policy statement for NAMBLA's privacy committee, which he had joined," and (2) "offered to host the  NAMBLA conference, and NAMBLA was unable to reschedule it after he revealed his identity." Nonetheless, neither of these items are reason to throw out the defendant's conviction, as the court correctly held (though it acknowledged that the latter action might be an infringement of the organization's First Amendment rights, and might under the proper factual circumstances justify a lawsuit by the organization).
One tidbit from the defendant, though I acknowledge that it's certainly possible that he was mistaken in his opinion: "Agent Hamer suggested that they form a travel group [for traveling to Thailand to have sex with boys]. [Defendant] Mayer responded with NAMBLA kept up pretenses of trying to change society when in fact its members only wanted to travel to meet boys."
Thanks to Howard Bashman for the pointer.