Steve asks what would happen if some states seceded from the US in this day and age. After some thought, this is what I've come up with.
I don't think any military action would be taken against the seceding states to "return them to the fold", as it were. I quite agree that the rest of the nation does not have the will to forcefully stop them. But the flip side of that is: no state or group of states have it in their collective will to secede in the first place.
Things have changed quite dramatically since the days of the Civil War. For all the pride and sense of citizenship that one derives from one's state, I think that allegiance to the Union as a whole has increased so much since the Civil War that it would be difficult to muster the sentiment to persuade a significant majority in a state or states to actually go through with secession. Many things have contributed to this, and it isn't necessary to go into them here in any depth, but among them are increased federal and decreased state power, the vastly increased abilities of transportation and communication, phenomenally increased interstate trade and the reorganisation of the military.
But, granting that it does happen, what would be the response? I think the most likely result would be similar in a lot of ways to the split of Czechoslovakia when communism ended. There would be hardships, some legal issues and a period of transition certainly, but I don't think there would be any bloodshed. Certainly none on a large scale.
A state like California that seceded would certainly be able to make a successful go of being an independent nation. Their economy is large enough, they would be able to adequately provide for their defence (though they would rely heavily for some things, like nuclear deterrence, on the US), they have a range of native industries that could be sustained and developed, a large coastline and varying natural and agricultural resources. As long as California didn't get too socialist, they would probably be fine as an independent nation.
Nebraska would almost certainly have a lot more problems. They would be, not only landlocked, but completely encircled by the US. This could lead to difficulties in trade, though their defense budget would probably only need to provide for police. More importantly for Nebraska, their lack of native industries besides agriculture would hurt them if they had to play by themselves on the world stage.
A side note for Greg's comment: California would actually make a net gain in terms of revenue by seceding. California is in the bottom ten in terms of money received from the federal government per dollar paid in federal taxes. Info is here. This might be offset by other things (tariffs, local tax hikes, etc.), but California would be a net gainer just looking at federal taxes and federal spending.