The problem with all of these e-currency schemes is that there's still a huge divide between the virtual e-cash world and the real "bills & change" world. I can't use Bitcoin to pay my rent or buy food. In order to do that, I have to go through a money transfer site. These sites are already pretty heavily regulated. I'm not even sure that any additional laws would have to be passed in order to make Bitcoin illegal; it might just take an administrative decision by the Treasury department.