Bitcoin is definitely taxable. If you sell all of your bitcoin for USD you pay a tax on the profits. Make sure to check with your local government tax office before failing to pay taxes.
but i think still there is no tax on bitcoin any where, still bitcoin is tax free, and the reason is that still not a single country consider bitcoin as legal currency therefore still there is no tax on bitcoin. may be in future when governments will accept bitcoin as legal currency then they will put tax on bitcoin.