I notice that I received quite a few responses, but I certainly did not intend my comment to be a troll.
I want to clarify that I think there is a difference between entering a piece of information as evidence and classifying that piece of evidence as proof of guilt. As with any other evidence, a piece of information can have varying degrees of implication depending on all of the other evidence compiled against a defendant.
Let's take Josh Stone's example: "Yeah, and since I own a gun, it must mean I killed somebody."
Suppose Josh Stone was indicted for murder. Obviously, if the victim was strangled to death, it is irrelevant that he owns a gun. If the victim was shot, it suddenly becomes more relevant, but we still need to know a lot more before we can decide that the ownership of this gun implicates him in any way (caliber, riflings, power residue, etc.).
Similarly, encryption is clearly a requirement for transmission or storage of child pornography (discounting stupid criminals). Thus, it is an important piece of evidence in a child pornography case. Much like the gun scenario, we still need to know a lot more about the possession of encryption tools (and as a side benefit - secure erasing tools) of a defendant. Was evidence of encrypted data found? Was evidence of erased data found? Does the defendant have a legitimate reason for using encryption?
Maybe I can describe my position here a little more concisely: I believe that evidence that has relevance to a case should be admittable. I believe that encryption tools have relevance in a child pornography case.
@Davi Oppenheimer: Thank you for the additional details provided by the case officer. I think it is clear that because the defendant was using a digital camera according to testimony that it is likely at some point the pictures made it onto his machine. I suspect that it is likely that the defendant could have erased (using the secure erasing features of PGP) the evidence and escaped detection. While erasing algorithms try to ensure that it is virtually impossible to reconstruct the contents of the original file, very few do this in such a way that it can't be detected that an erasing has taken place. However, this detection is quite difficult and could easily be overlooked.
@Don: I suppose one could say that all cases are decided based on circumstantial evidence. Because we can never be absolutely sure, we gauge our certainty of a defendant's guilt on the volume and consistency of whatever information we have available about the circumstances of an alleged crime. The use of PGP alone has never to my knowledge led to a conviction. In this case, I suspect the testimony of the young girl was far more heavily weighted. I totally agree that the use of PGP should not be admitted in every criminal case and that that would be a very dangerous precedent. However, until it becomes unnecessary for child pornographers to use encryption tools, I don't see the danger in admitting this type of evidence in child pornography cases.
I am not a lawyer, nor should this be construed as legal advice.