Funny, I would have attributed it to exactly the opposite cause - the average education level (or, more accurately, actual knowledge level) of the American population as a whole is going down. This is concomitant with a steady drop in the ability of American to use logic and reason. My church is in an extremely well-educated, economically above-average county and our congregation reflects this - even though we're a true-to-life fundamentalist Baptist church - the average IQ of our congregation would definitely be above the national average, as would our education level, and concurrently our ability to rationally defend the faith against all comers.
The problem is that most of the people who we're rationally defending the faith to - including many self-professed atheists - are not themselves capable of using reason. I've seen this first hand on many occasions. Emotion guides their thinking. They reject Christianity because it would keep them from doing something they want to do. Often, what they're rejecting is not even authentic Christianity, but a media-created caricature, which they've never bothered to rationally assess versus the real thing.
No, sorry, the problem isn't that the country's getting too "smart" for Christianity. If anything, it's just the opposite - the country's getting too dumb for it. The country's getting to the point where our publik skools are churning out hordes of mouth-breathers who can't think, can't reason, couldn't understand the ins-and-outs of theological doctrines well enough to truly decide whether they believe them. And the colleges are the same way. We need to understand that intelligence and the ability to think are not necessarily correlative with increasing education, even when you get to the advanced degrees, even when you're talking about "hard" subjects like the sciences. I've met plenty of science PhDs who struck me as being dumb as a pile of bricks, and left me wondering who in their right mind would give them a doctorate.