From an Atheist's Perspective
On the one hand, I understand the presuppositionalists' refusal to take part. They firmly believe that Christianity is the truth, that their god created this world and is the king over all of it. It would almost be an act of unfaithfulness to deny their god's sovereignty and adopt a perspective that assumes the very opposite of their faith. The Outsider Test asks them to act as if their god does not exist, something that they see as unfaithful.
Consider this map of world religions found here:

World Religions Map
Here's the key enlarged a bit in case it is too small in the picture:

Map Key
Now, from an atheist's perspective, it certainly seems reasonable to assume that, generally speaking, people adopt a religion because it is part of the culture of the geographic region they are from. In India, most people are Hindu. In the Middle East, most people are Muslim (though, some of the Christians here have made a good point that this is a state-imposed religion in much of the Middle East; I still think that it might be a stretch to assume that most Muslims in the Middle east only believe from coercion). In South America, most people are Roman Catholic. In the US, there is a close split between Roman Catholics and Protestants, but even these are geographically located.
I think that Christians, here, might even agree with this partially except in regards to their own Christianity. It doesn't seem unreasonable for a Christian to also assume that a person is Hindu because they grew up in a country of Hindus. I'm sure the reformed among us would say that these people have rejected the Christian god because they love their sin and that they have adopted a religion only to justify their rebellion, but it still seems reasonable to conclude that they adopted that particular religion because it was the popular religion of that geographic location.
I know that most Christians, here, believe atheism is a religion and we hold it like any other religious person holds their faith. I know that the Christians, here, believe that we are only atheists because we love our sin and we want to deny the Christian god's sovereignty over us. But I have to say, as honestly as I can, that it doesn't feel that way to me.
I feel that I am "unaffiliated" when it comes to religion. I feel that I am not a Christian because there is simply no good reason to believe and a lot of reasons not to believe. My moral life since leaving my faith is virtually unchanged. I have been faithful to my wife. I have a job that most people would not work that allows me to help poor, racial minorities in an infamously difficult inner-city setting. I have very close friends (all of whom are Christians, by the way--three career missionaries, two seminary students, one seminary graduate (i.e. aside from the career missionaries who are also seminary graduates)), a good family (who are also all Christian), etc.
My point is that it feels to me that, because I am unaffiliated, I am being "courted" by the various religions. The Muslims are recommending Islam, the Hindus are pushing Hinduism, the Christians are offering Christianity (in all of its various forms). . . Each of these groups say that theirs is the only "true" religion. I look at the map above, though, and I wonder if that same person would be arguing for a different religion if they had been born in a different place in the world.
In other words, it is hard not to dismiss a religious person's claim that their religion is the truth when it certainly seems, from my point of view, that that same person would be pushing another religion had they been born in a different part of the world.
In my post on presuppositionalism, I discussed relative, but objective, judgments regarding motion. I said that contradictory statements about motion can both be true given certain spatio-temporal frameworks. I can both say that my Guinness is moving and that my Guinness is not moving understanding that one statement belongs to one spatio-temporal framework and the other belongs to another one. I imagined that no one would really disagree with this, that it is easy to see the truth of a statement from a particular framework.
So, I'm willing to admit that, from the perspective of the Christian, the Outsider Test is an unacceptable act of unfaithfulness to their god. It asks them to reject the one they feel is the rightful ruler of the universe. I understand that (though, I would argue that if your god is truly a god, then he should probably be able to hold up under critical examination).
But can the Christians, here, not also admit that, from our point of view, it is certainly suspicious that world religions dominate geographically and that it is not unreasonable for us to conclude that religions are products of culture and geography, not products of "truth" and "falsehood"?