I ran across this video a while ago on what can happen to you if you don't pay your taxes. At some point during your Bitcoin career, the cryptocoin will be converted to fiat and that's how they will catch you.
https://www.youtube.com/watch?v=hf2fNEUPjQwOnce bitcoin is exchanged with our regular currency, that will be the time we can pay taxes. Much more when we are going to use that money like for instance in buying goods or anything, there is already a tax once you pay, example the value added tax. If ever the government accept bitcoin, then I am pretty sure there will be tax to be imposed.