People who didn't get my reference the other day to a "wave of pretty odd demographic hysteria" sweeping the country should definitely check out Johann Hari's review of Mark Steyn's America Alone. Alternative, you could read Steyn's book and experience the hysteria first hand. For a much less hysterical (and less racialist) take that still sees falling birthrates as a huge problem, pick up Philip Longman's The Empty Cradle.
It seems to me that the Longman version of the thesis, where population decline creates serious economic problems, at least could be true. To be convinced, though, I'd want to see more in the way of models that explicate the argument. Also relevant in this regard is Megan McArdle's non-alarmist take on the aging of the baby boom generation. In the greater scheme of things, replacing "maybe a proxy war will spin out of control and Soviet ICBMs will destroy major American cities" with "maybe a rising dependency ratio will lead to flat GDP per capita" as a problem scenario seems like a change for the better.