It cannot be shut down for the same reason that Bittorrent cannot be shut down: it is a peer-to-peer decentralized protocol. You need to learn about the basics... http://blog.zorinaq.com/?e=66
A similar attack was made against Wikileaks.
If you can't transfer your cash to Bitcoin, or your Bitcoin to cash, then the decentralized nature of Bitcoin itself is immaterial.
Unless you can turn your bitcoins into goods and services.
You'll have no way to convert electronic dollars into bitcoin. Saying that the government can prevent the conversion of cash into bitcoins is like saying it can prevent the conversion of cash into drugs.
For one, an often overlooked and dead simple way: you can sell goods/services and tell your customers you accept Bitcoin.
You can buy bitcoins for cash from sellers you meet face to face: https://en.bitcoin.it/wiki/Category:Directories
You can mine.
Etc.