My thoughts on the topic:
The rate of decline of theism in the U.S. is likely to increase during the next decade. Christians might even be in the minority by 2030. We might have some very interesting developments in politics resulting from the second major divide of society: Christians versus non-Christians, in addition to the current political divide between conservatives and liberals.
Of course the majority of the remaining Christians would be on the conservative side, but considering that about half of the current population would not consider voting for an atheist, there would be significant frictions within the liberal spectrum, with many liberals resenting the new political popularity of atheists in their midst. There might be attempts to form a separate liberal Christian party. The same might occur among the conservative atheists, who might be frustrated by Christians dominating their party leadership.
I feel very fortunate to be able to observe these patterns in my lifetime.
Besides the political shakeups, how would the culture change? The decline of theism might be seen as a kind of enlightenment, but other than bringing us closer to a separation of church and state and away from a general taboo on atheism, I doubt that it will improve much. The true intellectual awakening is going on right now, and will probably soon be over, as the main source of atheists shifts from critically thinking converts to indoctrinated children. People aren't getting any smarter; they still tend to think like their parents and peers, and the herd mentality will soon become the driving force behind the decline of theism, as later generations come of age in more secular environment. Some might be loathe to admit it, but "free thought" is just not a sustainable ideal. If atheism become prominent enough, the Christians might be seen as the intellectuals of society, having studied an area of which most were ignorant.
What about the causes behind the current trend? I think we might owe much of it to YouTube. Since it was founded in 2005, it has become a massively popular forum of opinions, and, I believe, one of the biggest influences on popular opinions regarding religion. I would estimate 5-10 million people have taken interest in the YouTube debates on religion, and that has boosted the popularity of atheist authors like Dawkins, Hitchens, Harris and Dennett (whose books on the subject have all been published in the last five years). In the absence of a similar source for loads of information and diversity of opinions, YouTube has actually become a repected debating ground, and one of the few places where atheists dominate.