Bitcoin is neither a commodity or a fiat currency, but has characters of both. The value is derived from the scarcity (since there will only ever be a finite amount of bitcoins, and before then there can only be so many at any given time, with no possibility of finding a new resource of them). The value is also derived from the liquidity perks that commodities and currencies have failed to offer, especially person to person, and over the internet.
If you bought a bitcoin with usd, the person you bought it from no longer has a bitcoin and would have usd instead.
For the IRS you would most likely have to report your cost basis in US Dollars, and at reporting time report a gain or a loss if you sold them for cash. In the absence of specific regulations (recognized foreign currency trading profits have a max tax rate subject to the 60/40 rule), you will use the short-term or long-term capital gains tax code. Short-term capital gains are treated as normal income and taxed in whatever bracket you are in. Long-Term Capital gains are treated in a lower max different tax bracket. Unless explicitly prohibited you can hold anything in an IRA or Roth account.......
1Just like any other currency exchange, then you find a person/company, agree on a rate, and you get bitcoins and the exchanger get dollars/euro/whatever you have. – Nicolai – 2013-04-01T22:21:53.400
So... lets say someone buys a lighter from me, it cost B2, how do i cash the money in order to buy more lighters? And i don't mean only in the USA i mean in more complicate economies, with hardcore money regulations, as Venezuela or Argentina? – elbreeder – 2013-11-19T16:16:29.023