FTA:
Taxation: How do governments collect taxes on transactions in Bitcoin? The answer is they don't, and they can't.Though many Bitcoin fans don't quite realize it, Bitcoin's radical transparency could in fact be a tax-collector's wet dream. Every public-key, balance, and transaction is public.
A government that wants to coopt and efficiently tax Bitcoin could announce the following policy, based on the idea of 'tainted' or 'clean' balances:
"You can make your Bitcoin legal inside our jurisdiction by registering your public keys. (No, we don't need your private keys.) All transactions between registered keys are great, just make sure they match up with your tax filings, because we can see the endpoints.
"You should not receive money to your registered address(es) from an unregistered address. If you do, you can file a disclosure with 30 days, identifying the origin and nature of the transaction, paying all applicable taxes, and then the balance is yours to keep."
"If you do not, the entire amount that's mixed with the unregistered-origin balance is subject to forfeit. Your registered keys with any such balance will be blacklisted, making your previously-registered balances unspendable until you come back into compliance."
Any above-ground business would then stick with clean balances. Both clean and tainted coins could circulate in the same blockchain, but rarely mix... and would have different de facto values. (Would a clean or dirty satoshi be worth more? I'm not sure!) Trying to buy above-ground things with dirty money would face the same 'laundering challenge' as today: making it look like legitimate income via front operations.
Except, the blockchain would be a perfect record of earlier related transactions. That means big-data traffic analysis, and occasional meatspace busts discovering the identities of unregistered keys, would create a very strong map of unsanctioned economic activity -- much better than is possible with physical cash.