Bitcoin is not a fiat currency with legal tender status in any jurisdiction, but often tax liability accrues regardless of the medium used. There is a wide variety of legislation in many different jurisdictions which could cause income, sales, payroll, capital gains, or some other form of tax liability to arise with Bitcoin.
Bitcoin may not fall under the law because it is not recognized at the official level. If this happens then the state will have to allow them to trade in bitcoins. Banks will be obliged to exchange bitcoin for Fiat. This is not happening. But as soon as we transfer bitcoins into Fiat, we are fully fall under the jurisdiction of the state.