Let's say they're determined to close it down. They could port scan every IP in the US and put people running the software in jail. Then you switch from raw TCP connections to Tor or something like that. The Government would need to be extremely repressive in order to stop BitCoin. As in putting people in Jail, for running some software.
That's the nice thing about being decentralized. At most they can close Mt. Gox, but then another mtgox will open somwhere else. In fact, Mt Gox is the Achilles' heel at the moment.
Don't get me wrong, I love the idea, but...
I realize that governments wont be able to enforce a world wide ban to any great effect. However the can still make sure that bank transfers etc. cannot be made to buy/sell bitcoins. And if you can't buy/sell them for real currencies on a site like mtgox, whats the use of mtgox? Also, if bitcoins cannot be tied to a real currency, how would you know the value of it? How could anyone decide on what the value is without an exchange?
Someone, somwhere, will allow you to exchange Bitcoins. Be it in Japan, Switzerland, Isle of man, or Sealand. It will boom and crash, but it's too convenient to completely fail. It just must fly under the radar long enough.
So I could just set up an account in Switzerland and trade bitcoins legally? And then transfer them back to my home country? Somehow this sounds a bit like money laundering.