Okay, but there are really two parts to your question. The first would be the FATCA legislation itself. I think what's going on in the United States is not so much on the constitutionality of the enabling FATCA legislation, but the way U.S. Treasury is trying to implement it with a number of intergovernmental agreements. I think Professor Christians was talking about these yesterday; she didn't understand what they were, but they are clearly in no way authorized by the enabling legislation.
Furthermore, I might add that it's amazing to me that Canada or any other country would sign an IGA obligating them to do anything when in fact the clear terms of the agreement obligate the United States to do nothing. As I understand it, the primary constitutionality has to do with the use of the sort of pseudo treaty provision, or whatever it is, to essentially enable law that isn't authorized by the governing FATCA legislation.