Barely half of Americans know what a Quran is according to a new survey from Pew. I wonder what percentage of those people think the president is a Muslim? Only about a quarter of Americans know that Indonesians are Muslim, even though Indonesia has the largest Muslim population in the world, which I guess makes sense since we haven't bombed or invaded them yet.
By the way, here's another interesting fact, atheists tend to know more about other religions than anyone else, with Jews close behind:

You might think this is because atheists/agnostics and Jews tend to be more educated, but according to Pew, the trend holds constant even when you control for educational attainment.
UPDATE: I think Jamelle's probably on to something here about why Jews, Mormons and atheists/agnostics are at the top--part of being a minority means developing knowledge about the majority as kind of a survival mechanism. But I think that may be overstating it a bit--if you're a religious minority and particularly if you live in an urban area, you're likely to live around and know people who practice a different religion other than your own.