The judgment is silent on the subject. Even the Soviet judge said nothing in his dissenting opinion. My reading of the testimony, which is in volume 17 of the Nuremberg proceedings, is that neither side really did a good job of proving its version. The judges did the right thing under the circumstances.
Among the interesting materials on Katyn and Nuremberg can be found the proceedings of the US Select Committee, which examined the evidence in 1952. Robert Jackson testified before the Committee. There is also a fascinating memorandum from the historian of the British foreign office.
The Katyn charge is often invoked in the victors' justice critique of Nuremberg. It is not the only atrocity committed by the victors, of course. To Katyn, one can add the firebombing of German cities, the use of the atomic bomb at Hiroshima and Nagasaki, and other Allied crimes. None of these has ever been properly punished. I've been intrigued by this question recently, not only because of the historic dimension, but for what we can learn in terms of prosecutorial decisions before our modern-day tribunals. In reality, the same types of problems remain with us. Personally, I do not attach validity to the charge that Nuremberg was flawed because it only prosecuted one side of the conflict. I will develop these ideas more thoroughly in my paper, which will be published along with the other proceedings at the conference later this year.
One of the big debates at the conference was the appropriate legal characterization of the Katyn massacre. Many of the participants were anxious to pin the label genocide on the killings. The argument here is quite a stretch, in my view, but readers of the blog can consult the whole debate in the videos of the proceedings and in the published version, when it appears.
Much attention was given to how the Katyn case might be litigated today. Currently, there are at least two cases pending before the European Court of Human Rights directed against Russia.