I dont think it will ever happen, if it will be still there will be so many ways to get pass those taxes lol.
I wouldn't say that it won't ever happen, though it's going to be hard to enforce taxes on each transaction as of course everything is going to be in bitcoin and people aren't going to pay this as most people are going to be anonymous to the government unless of proper enforcement of the wallets/exchanges. Only way I could see this is if they classed BTC as a currency and forced the wallets to listen to them on taxing a certain percentage (IN USD or the currency) all the time.
Tough to do, but if Bitcoin is enough of a threat to these people they'll sure as shit do it.