From "The Liberalism of John Paul II," by Father Richard John Neuhaus:
Liberalism, needless to say, is a wondrously pliable term. There is the laissez-faire economic liberalism condemned by Leo XIII in Rerum Novarum, and also by John Paul II. In American political culture that liberalism goes by the name of libertarianism, and, despite its many talented apologists, including Charles Murray (no relation to John Courtney), it has never acquired many adherents beyond what Russell Kirk called its "chirping sectaries." In the American context, libertarianism remains in the largest part a thought experiment for college sophomores of all ages.
There's something to that last sentence -- a lot, actually. I find it true of many blogospheric libertarians, and especially true with respect to the anarcho-capitalist branch of libertarianism. There, the state is rendered unnecessary, and therefore illegitimate, because the lambs who would dwell together in idyllic contractarianism believe that they can keep the lions at bay by closing their minds to the real world of real people and thinking of Lew Rockwell.
(Thanks to Keith Burgess-Jackson for the pointer to Fr. Neuhaus's article.)