Here is an interesting news story from CNN about governments and their ability to deny internet services during social unrest, like a revolution we are seeing in Egypt.
My Two Cents: Can the US shut down the internet. YES, and its not that hard to do. It all comes down to a very few Global DNS servers located in the US and other places outside the US. shut the servers down, and the DNS system is unavailable, the IPs would still work, but most folks don’t know that www.cnn.com is really 126.96.36.199. The government could also shut down like we see in Egypt all the ISPs and cellphone providers. For a long-term solution an emp pulse if used would destroy many router stations and communication satellites that provide internet services. If the US government really wanted to shut down the internet to the public they would go down certain paths of disruption based on the local social threat. I remember the days without the internet, they weren’t that bad.