Thus far, from my limited viewing of the media, the focus has been on the "political" advisability of using the phrase "ponzi scheme" while the words "unconstitutional" are largely ignored.
However there is a discussion taking place on As Maine Goes which takes a well examined look at the constitutional issue. I recommend reading at least the first post, if not further for an excellent examination of what the Supreme Court did wrong when it ruled social security was "constitutional" and what our framers actually said about the powers of the Federal Government in the Federalist Papers and beyond. John w k, the author of this thread has an impressive knowledge of constitutional law and history. Please click and read this excellent discussion and if you are so motivated- join it as well.