Greece is proving this by mandating that in 2011 and beyond all major purchases must be done electronically so that tax collection is automatic.
Sooner or later that rule will extend to all purchases and it will extend to all forms of barter and any other transaction. People will, of course, rebel against it and the government will, of course, try harder and harder to tighten the noose. At some point the concept of privacy and personal rights will just be chucked into the ocean and government will stop pretending that it thinks of citizens as anything more than livestock to be harvested. That will likely eventually happen in the USA as well but it will take longer. Meanwhile, the mechanisms for enforcing all of this are being put into place.
It might sound like fantasy or conspiratorial nonsense but it is exactly what I would do if I were some despotic SOB who just cared about controlling things no matter what. This is why government should not control money – the free market should. Government workers long ago stopped referring to themselves as servants. The people are now the servants and anyone who does not generate sufficient tax revenue for the master is referred to by those in control as a “useless eater”. It doesn’t have to be like this in the US but it will if we all just sit here and watch, cow eyed, as it happens.