I can see that when Bitcoin etc becomes more widely accepted and mainstream that governments will jump on the chance to tax it. Not seen any real ways that they would do this and implement it but i can see it happening

edit: i am in the uk and just came across this https://www.gov.uk/government/publications/revenue-and-customs-brief-9-2014-bitcoin-and-other-cryptocurrencies/revenue-and-customs-brief-9-2014-bitcoin-and-other-cryptocurrencies