People are already trading bitcoins for other property as a means of exchange just like money. Why would you need other money when Bitcoin is worldwide?
If the usage crosses certain limits, then the government will start taxing the exchanges and other avenues where the Bitcoins are being spent. The tax net is everywhere. It is not just possible to avoid it.
Taxation is dropping. People are finding ways around it, and even through it. The exchanges would be used less, or they would go underground.