There's an interesting post on GD forums here:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=389x5227391. It addresses really scary wingnut crazies speaking of, and in the worst cases, actually advocating violent revolution.
While I do believe that such unlawful rhetoric (I personally think these wingnuts advocating violent revolution need a visit from the Secret Service/FBI) has the potential to inspire the occassional unstable nutcase to commit an act of McVeigh-style violence, I highly doubt it will lead to an actual revolution.
However, I do think there is a small possibility that, in a future doomsday scenario, the United States may end up splitting into two nations, a "red America" and a "blue America" (I sure hope not). If (God forbid) the nation ever splits, it would least initially be peaceful, because of the existence of nukes. Rather, it would turn into a Civil Cold War, not unlike what happened with the United States vs. the Soviet Union during from 1945 to 1991.
With such a situation, there would be heavily defended border regions, similar to Panmunjeom in Korea or Fulda in Germany. An arms race in tanks, helicopters, aircraft and nuclear weapons would break out. Defections (probably from red to blue) would be commonplace. Spy vs. spy would be an integral part of a hypothetical situation; as would naval and air survellience, similar to US Navy operations monitoring Soviet ships. Blue America would have most major nations on its side, such as the United Kingdom, France, Germany, Russia, India, Japan and China; red America would probably be isolated and have few if any allies. Fears of an armed conflict would be very pervasive. And, with fundy crazies like the whackos we have advocating this insane crap running red America, the idea of them launching an all-out nuclear first-strike wouldn't be out of the question.
I agree that there's never going to be a conventional Second Civil War, and if these insane wingnuts tried to start one, they'd get crushed. It's highly unlikely and I certainly hope it never happens, but I think it's a lot more likely than a revolt by these whackos.