How can "they" prove I (ever) did?
And if proven, how can "they" tell how many I own(ed)?
And after "they" proved I owned x amount, how are "they" going to tax me?
The same way "they" prove and tax cash transactions.
By voluntarily keeping records then.
If you think "they" can't trace cash transactions without your help, you're in for a nasty surprise when you get audited.
Backtracking the blockchain to calculate how much btc I "owned" (at a(ny) given date) doesn't seem viable.
It's more viable than you think.
Let alone proving the "ownership" of privately mined btc.
They can just subpoena any mining pools you connected to for information on any accounts logged into from your IP address and the Bitcoin addresses associated with those accounts. Too easy.
How do "they" know I have this key?
There are far too many ways to list here.
Even if the wallet can be traced to me, they won't know how much in it is mine to spend without the key.
Blockchain.
So the "Rob me" billboard analogy, as mentioned above, sounds plausible when the law dictates I have to surrender my keys for "audit" purposes.
I'm not sure what you're talking about. "They" are pretty good at conducting audits without your cooperation.
I don't agree. btc is not money. Not unless my government accepts (tax)payments in btc it isn't. Until then it's more like gold and trading gold (as a service) is exempt of taxes. Only when you liquidise your gold you have to pay taxes (in a nutshell)
Until then btc should be considered LETS-currency, maybe.
Getting paid in gold is not tax exempt in any jurisdiction I know of.
Also, some of the btc I spent came from Satoshidice, which is a game of chance. Dutch law dictates you have to pay (income-) taxes over the net amount gained (wins minus losses) to retrace this (as a third party) is a next to impossible task. (to prove)
You clearly have no idea how SatoshiDice works. Every payout is directly linked to the wager in the blockchain. It is trivially easy to see prove how much you have won and lost on SatoshiDice.
The numbers in my bankaccount aren't money, the stuff the atm spits out is. Or at least in my point of view.
Tell that to any accountant and I guarantee he'll laugh in your face. Your point of view is wrong.