Not really related, but how about mining? What will happen once all the mining costs go down? What system will be put in place?
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...