Digging around the comments on Slashdot I found this nifty summary of just how, exactly, a nuclear-propelled spacecraft might work. Because the "bang plate" design has been around so long and is comparatively well understood, it's been the subject of probably dozens of different SF novels. "Footfall" is the most prominent one I can think of, other readers can likely think of others.

Of course, considering the fuss raised when NASA launched a space probe with 72 pounds of low-grade plutonium inside, trying to launch something using nuclear weapons seems... unlikely.

Still, it's good to know there are workable plans for really huge spacecraft in existence. If, say, a gigantic asteroid was going to splat the earth, I think most industrialized nations would be able to build them. Then we could leave Osama and his ilk behind so they'd be able to have a great face-to-face with Allah.