One thing that has not been mentioned in this thread is the first amendment defense. To spend bitcoins, you basically announce to the world that you have done so. To restrict bitcoin is to limit free speech.
This is where I think how bitcoin will survive many legal attacks. The exchange of bitcoins is the exchange of information, and as I understand it this is protected by the first amendment. Does anyone see a way around this by the government? Sure, there may be some specifics cases where the use of bitcoins for committing crimes will be blocked, but just the simple use of bitcoin can't be made illegal in the US without a Constitutional Amendment.