Nice post, though I kept thinking to myself, "Stop using the nuclear war example!" :^)
I think you are greatly mistaken with the adage that atheism is in someway connected with pessimism. Everything is relative, really. At what point is a legitimate idea pessimistic? Or on the other side of the spectrum, when is a mass delusion that an invisible guy with a bunch of winged angels flying about him, that can play musical instruments being "optimisitc"?
Whether a person is optimistic or pessimistic goes much further than their religious beliefs! While I understand how you try to say that pessimism isn't all bad, as in not having belief in a god, I hold great reservation with the idea that I'm pessimistic merely because I don't believe that a god exists. I'd say what you are calling pessimism, I'd call rationial thinking. Like I stated previously, is belief in a large invisible god optimistic even if it may be irrational?
"Pessimism is in response to fear, and that fear is often legitimate.."
Here is the problem. I, as an atheist, don't fear the unknown. Rather it is fear that makes man believe in god. The fear of the unknown. I don't fear the unknown to the extent that normal man does. Why do you think they need to create the idea of eternal life? Because they fear death!
Perhaps I was a bit misleading with my statement about living in the present. It isn't that we or I go around dreading the end is near. Its that we realize a couple of things.
1) Life isn't forvever
2) Life is impermanent
3) Time doesn't wait for you
As an atheist, the philosophy is that you have to put out the effort. It won't come to you. That isn't really pessimism. Pessimism would be believing in god and not trusting god to fulfill your needs. Atheism lacks the module of god, therefore they fill it themselves and enable their lives to their desires of experience. And you don't let small things get in the way. Take me. I went to a christian college about 300 miles from home. I moved over 700 miles away from home after graduating college. I oftenly go to places I've never been before, rarely settling for the familar. I'm only 25, yet I've completed in nearly 50 races in road running and triathlon, become very proficient relative to normal people regarding the Old Testament, I've got an engineering degree, I've been all over the Northeast US and southern Ontario, I can program in a few languages, study a bunch in science, and a bit more. This isn't the work of a pessimist. Its the work of a person that applies themself. Pessimism calls out, "Why bother?" Atheism requires one ask, "Why not?"