It depends on many factors.
I don't agree with the general statement made earlier that affiliate programs by themselves are dangerous. It seems to me that the scope of your potential liability would depend on how your association to the program is manifested in the operation of your websites. A key question then becomes whether you aware that the sponsor began to promote a CP site? and, if so, did you actively promote them. That alone would probably put a nail in your coffin, but failure to promote would be, in my opinion, a strong defense. Knowledge, however, puts you on notice of illegal activities, and therefore it could be argued that you assumed the risk of association with a criminal enterprise. That is a very dangerous argument because the probability is that in the matrix of all legal arguments a federal prosecutor will make, you can be sure that some form of conspiracy argument is likely to be among them, and it will be the most likely charge used to suck in members of affiliate programs.
Did you promote the CP site? If so, you are probably fucked. Direct participation is like putting a bullet in your ass legally. If you directly participate, and therefore directly promote the use of commerce to facilitate the transmission of illegal content, you will have little in the way of defenses.
There are some other vague issues which don't have much of an answer with regard to legal liability. Was your traffic redirected by the sponsor to the CP site? Were you advised in the terms and conditions of their affiliate agreement that they might redirect some of your traffice without your consent?
The only thing I agree with probono about is that by arresting you the negative publicity could fuck up your life, and the legal fees you would incur to defend claims regarding association with child pornographers could easily add up into multiple thousands of dollars.
As always, never easy questions, and never easy answers.
