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.