The US taxation system is based on taxes on self-reported income. Individuals/businesses are responsible for reporting their income accurately and honestly. Some forms of income are also reported on informational returns, which makes tax evasion regarding them more difficult. Bitcoiners appear to believe that "Bitcoin doesn't structurally generate informational returns so jackpot, no taxes ever" but there are any number of things which don't generate informational returns. You're still responsible for paying taxes; the government's main remedy if you don't is still auditing a very small percentage of returns and coming down hard on people who abusively underreported income.
Not a lawyer, but my understanding is that it is handled as a capital gains tax. So you buy some bitcoin, it fluctuates wildly over the course of a week, and you convert back to fiat. You then pay taxes on any earnings from that.
So if you bought 1 flubbercoin for 900 USD and sold it a week later for 1200 USD, you are liable for that 300 USD in gained capital.
Here is a marginally more technical description https://en.wikipedia.org/wiki/Capital_gains_tax
In terms of making sure you report those gains: What we see right now. Getting involved in the infrastructure to cut down on black money/money laundering.
But don't worry. I am sure The Followers of Satoshi will find a way to keep that currency volatile.
But what if you never have to translate it to a "real currency"?
Granted - bitcoin isn't anon - but let's suppose it was (or there existed a e-currency that was). If every transaction in the entire supply chain was carried out using such a currency, how would any government ever know (without inserting a MITM into the supply chain)?
An alternative thought: If you conducted all aspects of your life cash-only, how would the g-man "get you"? I suppose Al Capone could be brought up as an explanation (whatever was the reasoning there) - but there were other factors involved (they wanted him for other reasons, but tax-evasion was the easiest way - at least, according to what we've been told).
But as long as you conducted your affairs completely off-the-books, and only worked with vetted (or anonymous) others who were doing the same (so you never transacted cash with a government official or someone working for them) - how would they know?
Another thought experiment: If you conducted everything using barter (let's suppose that were possible) - would you then owe the government so many chickens or pigs as "tax"?
As for how to avoid The Man: Take a look at India. They lived that dream and likely will again in a month or two.
The Bitcoin wiki article on this is incomplete, but informative[0].
[0]: https://en.bitcoin.it/wiki/Tax_compliance#Are_bitcoins_taxab...