The thing is that anyone who understands the underlying technology behind the Internet would understand how it would be nearly impossible to "shut it down."
Most of the sites we visit every day are either located completely within the US or at least have mirrors (a server that hosts a complete copy of a site... mirrors are usually located in various locations around the world to provide redundancy and faster access times) located here. In other words... shutting down the links between countries would be possible... at least the major ones, but it would be nearly impossible to just "turn it off" within the borders... so many redundant links and connections exist... combined with the government's inability to perform even simple tasks efficiently... I just don't see it happening. Even assuming they could shut down all data flowing across the borders, since most of what we browse every day is within the US, most things would continue to work... remember, the internet all started right here in the US so the core infrastructure is still here... yes, things have spread out into the world, but none of it has actually been removed from the US.
I frankly see this as yet another case of legislators who don't understand technology writing laws for it (read: DMCA).
Even if you shut down all the major OC192, etc links running undersea here and there, you've still got ISDN, POTS, leased fiber, etc crossing the border between here and Canada everywhere... yeah, with the major links shut down it might make communication with parts of the internet beyond our borders nearly inoperable, but there is no easy way to completely cut us off.
