oh it could definitely happen, the actual problem is how enforceable the law would be. downloading movies and music through p2p means is considered illegal in the states, but its a pretty widely known fact that a large number of people do it anyways (and if you dont, you might actually be living under a very large rock). i dont doubt that a government with enough resources could potentially be able to set up an infrastructure solid enough to track bitcoin network related traffic, but the real matter lies in physically enforcing people to not use bitcoin. this would either mean mass sending of cease and desist letters, eventually culminating to physically sending law enforcement to a location. it's just not economically feasible, and there's too many existing ways to bypass detection.