"The Framers and the Faithful" is a lengthy essay by Steven Waldman, editor in chief of Beliefnet, published in the Washington Monthly. In it, Waldman argues that evangelical Christian leaders at the time of our nation's founding argued adamently in favor of separating church and state, in contrast to their spiritual descendents in the modern evangelical movement. Those early evangelicals were members of a minority movement persecuted by the established churches of their day. As such, they viewed a state-established church, along with any other government-supported religious activity, as a threat to their survival. They also saw state-sponsored religion as leading to the moral decay of both clergy and laity.
So, what has changed in the evangelical movement to cause such an about-face today? Waldman doesn't directly address that question. IMHO, the difference is simple: today's evangelicals have much more political power and social influence than their brethren of yesteryear, so any state-sponsored religious activity would likely be in line with their beliefs. What too many of us Christians forget, however, is that political winds change. Once government becomes excessively entangled with religion, it will be difficult to disentangle it when different political views predominate. In the long run, the separation of church and state is most important for protecting churches from excessive government influence rather than the other way around.