Let's Bitcoin takes off. Say you lived in the US and had an online service that dealt in the equivalent of ~20,000 USD or more income for you (personally). What are the laws for income tax on a currency like this? Can you get in trouble with the IRS by simply having tons of BTC that the market values at (currently) almost 5$ per BTC?
In the US, Bitcoin is treated as a commodity and is thus not taxed unless you want to cash out to FIAT.
It is only then that you're forced to declare your income and be taxed on it. Bitcoins themselves are not (and cannot be) taxed.