Not to mention, I'm not even sure what question is being asked or should be answered as the OP has given us three seperate ones.
From the subject: "Do you think this country was founded upon Christianity?" This question seems to be asking us if Christianity, and its principles, helped lead to the foundation upon which the country was built
From the Poll: "Do you think the U.S. was intended to be a Christian Nation?" This question seems to be asking us if the foundered INTENDED for the U.S.'s population to be that of a Christian Nation. IE, that the majority of those living within the confines of the general border of the "United States" would share a common bond of a similar base religion amongst them, with that religion being Christianity.
From the Original Post: "I hear all the time that 'this is a Christian Nation'. Do you think this is so?" This question seems to be asking whether, regardless of Founder intent, we are a Christian Nation (described above)
Funnily enough, the question most answered in this thread has been "Was the intent of the Founding Fathers that the United States should be a Christian State?" and/or "Is the U.S. a Christian State?". However, save for possibly through implication and the assumption that the OP doesn't know what "Nation" actually is defined as, this question was never even asked.