The more interesting information, here, is that 51 percent of registered voters don't "Identify Obama as Christian."
This isn't because they think he's a Muslim, though. There are some who think he's Muslim, but the number that identify Obama with Islam has been pretty consistent and relatively small, upticking only slightly from 12 percent right before the 2008 election to 17 percent today.
It's possible to read this "don't know" as simply the safe version of saying he's Muslim. Some Republican officials have taken this stance of allowing doubt and encouraging distrust without actually saying anything directly. Like, "he says he's a Christian ... (but I don't know)," etc.
There's another way to read this question about Obama's Christianity, though, which has nothing to do with rumors he's a secret Muslim.
It's possible 31 percent of registered voters don't know if Obama's a Christian because they don't know if liberal Christianity is really Christianity.
Consider: a year ago there was an intra-evangelical fight about the doctrine of eternal damnation, after megachurch minister Rob Bell wrote a book questioning some aspects of that idea. The terms of the fight were, to a large extent, could one still be considered a Christian if one did not believe in hell? This is not an exaggeration. Though it was portrayed as a conflict over this doctrine, it was actually about who gets to count as Christian. So John MacArthur wrote, e.g., "Is Rob Bell truly a Christian, or is he one of those dangerous deceivers Scripture warns us about repeatedly (Acts 20:29; 2 Corinthians 11:13-15; Colossians 2:8; 2 Peter 2:1; etc.)? It's a fair—and necessary—question." Kevin DeYoung, who started the extensive attack on Bell, wrote that "At the very heart of this controversy ... is that we really do have two different Gods" and "Bell’s vision of heaven and hell doesn’t work because his vision of God is false."
For people who take this position that Christianity is only Christianity if it includes a strong vision of eternal damnation, Obama's not going to count as Christian.
In one of his more extensive statements on his faith, for example, the then-Illinois state senator said,
"I find it hard to believe that my God would consign four-fifths of the world to hell. I can’t imagine that my God would allow some little Hindu kid in India who never interacts with the Christian faith to somehow burn for all eternity. That’s just not part of my religious makeup."DeYoung, and those like him, would not likely conclude from the above that the president is a Muslim, but they wouldn't say he's a Christian, either. Not a "real" Christian, anyway.
Thus: "don't know."
It's a kind of yes and no answer, as in yes he's a Christian in the sense he identifies as such, but no, he's not really a Christian, because he embraces a "phony theology ... not a theology based on Bible." The "yes" is about a descriptivist account of Christianity, which allows that people are as they say they are, and the "no" is a prescriptivist account, which holds there's an essential definition of Christianity, and someone can say they're Christian without meeting the objective theological standards of being Christian.
It's possible that nearly a third of the country's registered voters are just confused and have been tricked by conservative commentators into suspecting the president of the United States is other than what he claims to be. A more reasonable explain, though, it seems to me, is that this 31 percent recognizes Obama is, as he says he is, a liberal Christian, but they don't know that that Christianity is really Christianity.
For them, liberal Christianity is in a confusing neither/nor space that's kind of hard to name, especially when going back and forth between descriptivist and prescriptivist definitions of "Christian," and that's exactly the gray area where Obama and his faith fall.