Or as I wrote in my blog http://www.dewsofquietness.blogspot.com

Which I just had a look at... sigh!

Your assertion that atheists "are a diverse and freewheeling group, united by our lack of religious belief" is something of an oxymoron since by definition (here:http://originoflifefairness.org/courtdefinedreligion.html and end of second paragraph here:http://www.investigatingatheism.info/definition.html)atheism is a religion.

Let's see, the first link is a Xian group that misinterprets a court document that talks about how atheism is an idea that touches upon religious concepts, and can therefore be considered a "religion" for purposes of non-discrimination in the first amendment. The second link doesn't support your statement that atheism is a religion.

They just don't believe that a god exists. This means that, by default, they DO BELIEVE that there is no God.

Incorrect. Theists have not met their burden of proof that god exists. Therefore, I do not share their positive belief. Denying that your god exists does not necessitate that I have presented a positive belief of my own. There may be mustard in my refrigerator, but I don't know. You could present what you believe to be evidence that I do have mustard in my fridge, but I doubt that you could meet the burden of proof to convince me it is there. Does this mean that I believe there is no mustard in my fridge? No, of course not. There may be mustard there, I don't know. But, sans evidence, I'm not justified in claiming that I believe there is mustard there.

But this is like saying that "black", which physicists define as the total absence of color, is not a color.

and as has been said hear several times "If atheism is a religion, bald is a hair colour". Or similarly "atheism is a religion in the sense that not collecting stamps is a hobby".

They claim that they simply do not believe in a god or higher being. Well, so much for Atheist logic. It is logically impossible to have no beliefs.

This is a non-sequitur, non-belief in a deity does not mean to have no beliefs. However I suspect you are conflating a faith position - "I believe in a god despite or possibly even because there is no rational reason to hold such a belief" - with an epistemological belief in observable falsifiable facts - "I believe the chair I'm sitting on is made of wood" or "I believe the theory of evolution is supported by the evidence".
The most you can say about atheism as a religion is that it has a philosophical apposition to supernatural phenomena up to and including the existence of a deity.

I would only add that I think that while the FFRF's Olympia sign was wisely provocative, one can be provocative without solely engendering rage. The UK bus ads are a good example of a message that was controversial, but has sparked thoughtful debates and questions. The FFRF sign, I think, sparked almost entirely vitriol. Perhaps that's what they wanted, but I don't think it was the best strategy overall. I want religion criticized as much as the next guy, but this is why we have tactics of persuasion and nuances of rhetoric, and not simply megaphones.

I don't concede that it provides a source of meaning. The source of meaning doesn't come from religion but from ourselves, whether we incorrectly ascribe it to religion or not.

I also don't concede that it helps with moral behavior, for a couple reasons. I don't see anything moral in only doing good to satisfy some sky daddy. The end result is good, and the alternative might be worse, but the ends don't justify the means.

I do concede that it satisfies whatever need people think they have for ritual and ceremony, although I'm not so sure that it's been established that such a thing actually exists. If it does, it may be a by-product of religion itself, meaning that religion created a need that it then fulfills, which kind of undercuts the beneficial aspect.