I think if you take any religion (eg. Christianity in one of it's flavours) you are going to have practicing and non-practicing members. I attended a Christening on the weekend where one of the Godparents complained the whole time about having to sit through a service, and what a load of bs it was. Yet they got up and recited the prayers, etc.
I would imagine that whilst they might identify themselves as a Christian, the Priest as the front of the service might have been a little more circumspect. In any case, there are a lot of people following religions to some minor degree because they were born into it, it's traditional, etc.
They would generally be the ones least informed about religion. I could never be a Godparent to a child, which is a discussion I have had with friends and family more than once. And in my own way, I think that shows MORE understanding and respect to their religion than they themselves would be showing by getting an atheist to stand up and lie in church...! There has been pressure at times, although I am lucky to have friends and family who respect my beliefs, even though they have very little comprehension of them I think.
Incidentally, the Priest went out of his way at the start of the service to welcome people of other faiths, and non-believers, and invited them to participate to the level they felt comfortable. I thought it was a nice touch, given the mixed crowd you always get to Christenings (or weddings, funerals, etc)
Anyways, I did have a point somewhere. Umm...oh...I don't think someone identifies themselves as atheist unless they have really thought it through. Not sure exactly what they would identify themselves as. Just someone who doesn't follow a religion, I guess. But to actually say 'I'm an atheist' is a sign of having considered the issues at hand. Perhaps comparing atheists to practising members of a religion would be a fairer comparison.