The above sources says it is based partly on a screed by Ben Stein, which is easy to believe considering other things he's written.
Apparently the White House referred to Christmas Trees as Holiday Trees for the first time this year which prompted CBS presenter, Steven Levy, to present this piece which I would like to share with you. I think it applies just as much to many countries as it does to America.
I have no idea where the concept came from, that America is an explicitly atheist country. I can't find it in the Constitution and I don't like it being shoved down my throat...