When it comes to terrorism, the Internet is not too far removed from the telephone. Both allow for free and open communication. Both can be monitored (more or less) by law enforcement (one more easily and more often than the other, perhaps). Both technologies are in and of themselves amoral. Both have certainly been used in the commission of petty misdemeanors and felonious atrocities. Both have become too important to commerce to be removed from society without a much-improved replacement.
As far as travel is concerned, while it is certainly easier today than ever there are still significant hurdles to the cultural migrations Sterling mentions--short of using the U.S.-Mexican border. Travel is expensive, but necessary for terrorism to succeed. Travel is less necessary for commerce (I'm talking about the movement of human beings, and excluding the shipping of merchandise).
I don't have any great insight in how to resolve this particular issue (both freedom and liberty can be used against themselves--see Rousseau), but I think this article is worth some pondering.
- R.L.