The current way of exchanging Bitcoins is not decentralized at all. I guess the first thing governments will do, if they ever consider Bitcoin a real threat, is to try to shutdown the major exchanges.
If they succeed, which still will be hard, I'm not entirely sure Bitcoin will continue to be that usable in spite of other decentralized ways of exchange like http://bitcoin-otc.com/ or http://localbitcoins.com
Lots of Bitcoin businesses are online services. If US businesses are banned from offering Bitcoin services, this will just bring more customers to services operated in other countries.
If they succeed, which still will be hard, I'm not entirely sure Bitcoin will continue to be that usable in spite of other decentralized ways of exchange like http://bitcoin-otc.com/ or http://localbitcoins.com