Wherein I Disagree With Ed About Atheist Politicians
Speaking for myself, there was never a time when I would have voted for a conservative atheist over a liberal Christian candidate. I think this is an incredibly easy question to answer. I simply don’t see what someone’s religious views have to do with who I vote for, especially since openly atheist politicians are nearly as rare as albino unicorns. I couldn’t possibly care any less whether a politician believes in God or not; I care what they believe about the issues that matter. I just don’t see why this is even a remotely difficult decision.
Ed's heart is in the right place on this one and, in an idealized world where people were able to adequately compartmentalize their beliefs, he'd probably be right. However, in a non-idealized world a person's religious beliefs, or lack thereof, are likely to influence their preferred policy positions, so to disregard their religion entirely seems like an act of willful blindness.
Consider that a politician who holds views that ey knows are unpopular may conceal, equivocate, obfuscate, and even outright lie about them in order to win election. We've seen a lot of that this election cycle in regards to the Romney Tax Plan or Ryan's on again/off again relationship with Ayn Rand. On the religious front there's Mike Huckabee, who doesn't seem particularly crazy (for an evangelical), except when you start digging a little bit you find out that he may be a dominionist. Surely that point is relevant in assessing the sorts of policies that he'd be likely to endorse if he were to win office? To address Ed's criticism directly, religious belief can be a valid proxy in a world where politicians aren't forthright about "what they believe about the issues that matter".
There is also a meaningful discussion to be had about how atheists differ from theists. Ed is right that openly atheist politicians are exceedingly rare, in which case questions of belief usually fall along the lines of "too much/not enough" and "right god/wrong god". These, I concur, are totally useless.
However, there are real, material differences between theists and atheists. The former has, by definition, a metaphysical commitment to the supernatural while the latter (probably, but not necessarily) does not. All else equal I'm more likely to agree with/trust the judgement of an atheist by virtue of the nature of eir metaphysics.
Now, as to the original question, it essentially asks which axis (belief/unbelief vs. liberal/conservative) should be weighted more heavily when evaluating a candidate for office. It's objectively a bad question because the axes aren't independent; there's a high degree of correlation between belief and political inclination. That's the major reasons why conservative atheists are so rare. It also treats liberalism/conservatism as a single axis for purposes of evaluation when, in reality, political ideology is multivariate.
More subtly, it also relies on a dichotomy of "liberalism good/conservatism bad", which is completely baseless as an abstract proposition. There are lots of different beliefs systems which qualify as liberal, some of which are useful and some of which are foolish and wrongheaded; the same is true of conservative belief systems. I'd rather elect a Burkean conservative than a Titoist.
Those objections noted, what I'm most concerned about in a politician, first and foremost, is that they be reality-based/evidence-driven. Given the discussion of metaphysics above I would pick the conservative atheist on the grounds that they are more likely, all else equal, to be so.