There are plenty of ways to convert US dollars into Bitcoin, without exchanges.

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.

> I think the basic principal is that it will be unusable within the USA, because you'll have no way to convert US dollars into Bitcoin.

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.

> 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.

people buy bitcoins for cash in person today. if anything, preventing financial institutions from accepting bitcoins makes the transactions harder to track.