Technically Athiesm is not a religion, but in the common sense of the word, along with it's many parallels to religions, it might be. For example:
Religions have prophets/respected leaders: Jesus, Budda, the Pope, Billy Graham.
Athiests have prophets/respected leaders: Hitchens, Dawkins, Darwin.
Religions have a theory as to the beginnings of existence: Creationism, Great Spirit
Athiests have a theory as to the beginnings of existence: Evolution, Aliens
Religions can condemn those who disagree with them.
Athiests can condemn those who disagree with them.
Religious people go about trying to get people to convert to their belief system.
Athiest people go about trying to get people to convert to their belief system.
The list goes on and on, but the major differences are that religion has an organized system, churches, and worship. While Athiesm is largely independent, has very few places of congregation, and does not worship ( although the case may be made, that scientists are worshipped in a sense) Is this enough to say that it is not a religion? Maybe. But given it's belief system, and tendencies, most people would consider it a religion, not in definition, but in practice.
As to the issue of the cannon advertisement, I have no problem with the athiests paint job. However, in yet another parallel to religion, their choice of wording was delibrately meant to insult those with different beliefs. Now, I have no problem with that, as long as you don't complain when the opposing group returns that action. That goes for both religious people and athiests.