People are upset, and rightfully so. It's kind of an asshole move.
The billboard, along Route 495 outside the Lincoln Tunnel in North Bergen, New Jersey, was paid for by a group called American Atheists. They claim the billboard isn't evangelical in nature, that it's there to encourage other atheists to be more vocal around the holidays and to "attack the myth that Christianity owns the solstice season" and to "raise the awareness of the organization and the movement."
Fair enough, but is it necessary to attack people's beliefs in the process? Why not just focus on celebrating your own beliefs rather than pissing people off by putting theirs down? Ridiculing a belief system that, for many people, is the basis for their entire life philosophy is not a way to win friends or influence people; it's a way to act like a stereotype.
I was raised pretty devoutly Catholic, attended a Catholic university (Go Irish), and after college realized that I'm an atheist. My family is still Catholic and many of my friends still attend mass, and every day, I struggle with Not Being A Big Fucking Dick About It (and I often fail and end up acting like a jerk, and then I feel horrible). This is an example of a group of atheists losing that struggle.
Chris Hitchens-like atheists and agnostics would disagree with me (and then tell me that it is impossible for me to be funny, as a woman), but, in my experience, I've found that confrontational atheism isn't productive, just as aggressive proselytizing on the part of religious groups isn't a good way to endear the religion to the populace in the modern era. Many of the holidays that are celebrated around this time of the year are religious in nature, and part of existing as a decent person in this world is letting people go about their daily lives in peace.
"Atheists Billboard Calls Nativity A Myth" [FoxNY]