If I had more time on my hands: (1) Launch a Bitcoin capital gains/losses tax approximation application (the "App") in beta. User keys in their Bitcoin addresses. App searches the block chain for the user's entry and exit times. App searches exchanges for the most favourable pricing source. App then returns an approximation of taxes owed/to be credited (e.g. in case of coins lost at Mt. Gox). App has a prominent disc…
How will you know which output is spent on purchasing something for bitcoins (a taxable event), and which is change, sent back to the user?
You'll see a bunch of transactions like these:
Redeem 3.4784 BTC from output 1 from tx 23e23f23f23be52bef98a8b... Send 0.879 BTC to 1CjPR7Z5ZSyWk6WtXvSFgkptmpoi4UM9BC Send 2.5993 BTC to 1NxaBCFQwejSZbQfWcYNwgqML5wWoE3rK4
Has the user spent 0.879 or 2.5993 BTC on something?
Or has he just sent money to another wallet, or deposited with an exchange?
The index number of the non-change output in a transaction is intentionally randomized, so it can't be figured out, from blockchain data, which is the payment and which is change.