Care to explain? Taxes apply to capital gains, which are only realized when you sell. Not to holdings. People who have large coin balances don't owe taxes unless they sell coins, and then the tax is a percentage of the sale.

The logic is that people who mined or sold bitcoin last year (and at the time didn't know how it would be taxed) now need to sell more bitcoin to get the cash to pay their taxes, and all that selling depresses the price. This is a plausible mechanism, but I think it's clear that news from China is having a much larger effect on the price right now.

False.

Taxes also apply the moment coins are mined. See question Q-8 in the IRS notice.

That's fair, but it only applies to miners, not speculators who bought their coins.

Also - does this apply retroactively to coins mined years ago? I.e. Does satoshi now owe millions of dollars in taxes?