This anonymous question is from the Ask Me Anything page. I've been in Chicago on vacation so it took me a bit longer than usual to answer it.
Well, I think we should start by defining our terms. We'll start with the easy and work up to the not so easy.
Atheist You would think this is easy, right? Sure, Atheists don't believe in god(s) but that leaves a lot of folk under the big umbrella of Atheism. Atheists may be: believers in karma (the moral Law of the Universe), alien abductions, buyers of lottery tickets, or just no nonsense empiricists. We are a very diverse group.
Buddhism Buddhism is based on the teachings of Siddhartha Gautama who lived in sixth century BCE India (click here for link). His teaching may be summed up as: People are miserable because they attach their sense of who they are to impermanent things like: one's physical body (it will wither and die), a job, and/or social role. The trick to not being miserable is to cultivate a sense of non-attachment. Non-attachment is attained by discipline and/or rituals. That leads us to the two main branches of Buddhism.
Theravada Buddhism has been called Little Boat Buddhism (click here for link and look under Buddhist Faith). This school focuses on a hardcore ascetic lifestyle. If you see a statue or picture of a thin Buddha then that guy is usually a Theravada Buddha. This form of Buddhism is common in southern Asia. These guys are typically Atheists.
Mahayana Buddhism Is more like "Buddhism for the masses". While Theravada Buddhism asserts that only a few hardcore badasses get out of the cycle of misery and become enlightened, Mahayana states that everyday schmoes like you and me can become enlightened too (click here for link). Mahayana is known as Big Boat Buddhism and is popular in China and Japan. Zen, a form many Westerners are acquainted with, is in the Mahayana branch. As Buddhism migrated from India through Tibet and China it picked up folk aspects of religions already there. Those folk aspects include supernatural entities and deities. Zen, it should be noted does not recognize deities.
So you are asking yourself, "Just answer the damn question Andy!"
What I'll say is that you CAN be an Atheist and be a Buddhist.
But being a Buddhist does not automatically make you an Atheist.