Atheism is a religion.
http://dictionary.reference.com/search?q=religion
"reˇliˇgion ( P ) Pronunciation Key (r-ljn)
n.
1. a. Belief in and reverence for a supernatural power or powers regarded as creator and governor of the universe.
b. A personal or institutionalized system grounded in such belief and worship.
2. The life or condition of a person in a religious order.
3. A set of beliefs, values, and practices based on the teachings of a spiritual leader.
4. A cause, principle, or activity pursued with zeal or conscientious devotion." (note the "or" in between "zeal" and "conscientious devotion")
Atheism is by definition a religion if we go by the forth listed definition, as it is (sometimes zealously) devoted to the lack a God and the supernatural.
Another arguement for this view is the idea of Zero (0).
Zero is the idea of the lack of something. If the variable has no value, we define its value as 0. 0 is the lack of a number, yet it is still a number.
Similarly, Atheism is the lack of a religion, yet it is still a religion.