Looking at dictionary.com I see atheism fits non of the definitions offered.
1. a set of beliefs concerning the cause, nature, and purpose of the universe, esp. when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs.
It's already been pointed out that atheism does not fit this definition. Atheism doesn't offer a set of beliefs concerning the cause nature and purpose of the universe. It doesn't involve devotional and ritual observance, or moral code. And by definition lacks the belief in a superhuman agency or agencies.
2. a specific fundamental set of beliefs and practices generally agreed upon by a number of persons or sects
Atheism is the lack of belief not belief. It holds no practices generally agreed upon by a number of persons and has no sects.
3. the body of persons adhering to a particular set of beliefs and practices
Again atheism is the lack of belief and has no particular practices.
4. the life or state of a monk, nun, etc.
I'm not sure if this definition even fits religion, this is more about being part of a religion then being religion itself. An atheist can still be an atheist and be part of a religion that has no deities, but that doesn't make atheism the religion.
5. the practice of religious beliefs; ritual observance of faith.
Again atheism the lack of belief, most atheists don't hold faith.
6. something one believes in and follows devotedly; a point or matter of ethics or conscience
The one that has been waved around here. But again, atheism is the lack of belief and most do not hold devotedly to atheism. Atheism holds no points of ethics or conscience.
7. religions, Archaic. religious rites.
There are no rituals in atheism.
8. Archaic. strict faithfulness; devotion
As pointed out above most atheists don't hold faithfully or have devotion to atheism.
9. get religion, Informal.
a. to acquire a deep conviction of the validity of religious beliefs and practices.
b. to resolve to mend one's errant ways
being this is an informal use of the term it really doesn't hold validity in this context.
But A. many atheists don't hold a deep conviction to the validity of religious practices.
B. I would suppose man atheists might hold to mending errant ways based on reason, but this would only indicate reason as a religion in an informal manner rather then atheism. With atheism in this case simply being a product of that reasoning.
So no matter what definition you use from dictionary.com atheism doesn't fit as a religion.