I believe that at the moment this is very difficult, but in the future the income tax has some change because of the BTC. Now, how and when exactly this can happen is difficult to know.
The government will not allow to lessen their tax collection, it's their lifeblood so they will do necessary things to address this possible problem.
Bitcoin is just a system that we can use to pay but the it will always end up with exchange sites then to our bank accounts, with that flow they know what to do in order to enforce collection to us, because if they cannot there is no reason of supporting bitcoin and they can easily declare a ban on bitcoin.