Is Atheism a Religion?
Atheism does not hold any religious beliefs, and does not adhere to any religious doctrines. So no, atheism is not a Religion (capital 'r') in the classic sense. Atheism isn't a set of beliefs, it's a lack of beliefs. It's the default state. All infants are atheists because they haven't learned otherwise. It's only after introducing beliefs to the child that it will become a member of a religion. If the child is never taught religious beliefs, they would remain an atheist.
Still, I think that atheism is a religion (lowercase 'r') as a demographic set. If there was a survey that asked me "What is your religion?" I would answer "atheism." It is a valid label for the demographic group I belong to. As a label, atheism is a religion in the same way that zero is a number. Zero has no value, but it is still a number.
Is Atheism a Belief?
No. Atheism is the position of having no beliefs. While an atheist could declare their position to be "I believe there is no God" (sometimes referred to as strong atheism), describing the lack of something should not be confused with an actual belief. This is simply because there is nothing there to believe. Imagine two explorers charting a never-before-seen, uninhabited island. One says to the other, "What race do the people belong to?" The second explorer says, "There are no people. It's uninhabited." The first one nods and replies, "They must be indigenous, then." This is ridiculous because the lack of people can not be described as a type of people.
Is there such a thing as a Fundamentalist Atheist?
No. The vast majority of definitions available for the term Fundamentalism reveal its overwhelming connection to religion and the bible. These definitions use phrases like "movement in American Protestantism," "infallibility of the bible," "religious movement," "opposition to secularism," "adherence to the theology of..." The only secular definition I could find was strict adherence to any set of basic ideas or principles.
Now, atheists do have a basic principle: we do not believe in a divine being. But it is not something that can be adhered to, strictly or otherwise, because it is a defining principle. There is no spectrum of adherence to this principle among atheists. (thought when applied to people in general, of course, you get the theist/agnostic/atheist spectrum) It is the equivalent of a theist's belief in a god as their defining principle. You can't call every Christian that believes in God a fundamentalist, because that belief is one of their defining principles.
Want more? Atheist Revolution recently had a great post (one of the inspirations for this one) where he has tried to investigate Atheist Extremism. Two of the terms he (? I think vjack is a he....) looks at are "fundamentalist atheist" and "militant atheist."
In a nutshell, religious fundamentalism is about adherence to a particular doctrine, atheism has no doctrine, and therefore, there can be no fundamentalist atheism.
The Uncredible Hallq looked at fundamentalist atheism and militant atheism fairly recently as well. He decides that it is used by theists who either want to slam atheists, or are using the term to mean "hard core."
But outside slamming atheists, fundamentalism has always mainly applied to people who believe their given holy book is infallible. Sometimes, this is taken to the point of people becoming convinced that "God exists because the Bible says so" is a good argument.
And finally, I will direct you to The One With Aldacron, who agrees with The Uncredible Hallq that the term is used as a replacement for "hard core" or, as The One phrases it, "hardline."
The term “atheist fundamentalist” is an oxymoron. Fundamentalism arises from faith. Atheists have no faith. So there is no such thing as an atheist fundamentalist. There are some militant atheists out there who love to provoke confrontations with the religious. But I think they are rare. Hardline atheists are those who stand up for atheism and denounce religion for what it is.