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It's a telling move. That is, atheism has started its own evolution from a philosophical viewpoint to a fundamentalist movement. When a historical figure -- who made no claim to deity -- can be lauded by his adherents 200 years later as one who should be praised, then a certain deification is blooming. No matter how small, or how ironic, or how innocuous it was intended to be, the fact remains that Darwin is now being elevated to a level he would probably reject were he alive.
On top of all that though, atheism is not about evolution. The evolutionary model is simply a naturalistic understanding of the emergence and development of the world and its species. Atheism is quite simply a lack of belief in deity, and/or a rejection of the supernatural. So while evolution provides an understanding, that understanding is simply a tool in a much wider perspective about the nature of reality. Atheism, if it is properly understood (and I don't claim to have exhaustive understanding) does not limit itself to evolution as its entire epistemology.
That being said, it would seem to me that the catchy notion to 'evolve beyond belief' ends up in a bit of a conundrum: why would an atheist believe evolution? I have reasons why I, as a theist, believe evolution. But those reasons come with a price: a belief that my reasons are reliable, that rationality is reliable. So if these particular atheists, the ones who are touting the 'Praise Darwin' billboards, are encouraging us to 'evolve beyond belief', how is it that they believe anything of what they're promoting? How do they know their rationality is reliable? They've taken the floor out from under their own feet by implying that human beings are more evolved, genetically superior, when they lack belief. But how can they say that and believe it to be true without also disbelieving it?
The only way it can be true is if it is false. And the only way it can be false is if it is true. Essentially, the billboard means nothing. However, the connotations it brings about will addle people's brains enough that it will seem like something.