So what happens if the US Federal government determines that it is illegal for any US business to accept bitcoin?  Could the currency still survive?  Would there be a use for it?  Since it would only be illegal to conduct business transactions with bitcoin, could it be used a currency between individuals?

Could the US Federal government merely make it illegal to trade bitcoins for USD? If so, how would it be enforced? Could a tax audit reveal a bank transaction to or from an exchange?  How would this effect the value of bitcoins?