There's an official federal holiday named "Christmas." Every single store in the nation has Christmas sales. Many non-religious people celebrate Christmas in the US (I celebrate it and I'm an atheist). I think it should just be simply "Merry Christmas" simply because it isn't a religious holiday - although religious people are free to celebrate it as one. The rest of the nation doesn't have to.

EDIT:

Wasn't it said though that Jesus was actually born in April.. not December.. that Xmas is actually a pagan holiday or something?
Most Christian holidays are based off of pagan holidays, such as Easter. Bunnies and eggs were the symbols of the common pagan spring celebration of fertility.