Which is news because he's something of a rubfuck mascot for digital libertarians.
He has a blog post at antipope about it and a handy link farm as below:
Mining BtC has a carbon footprint from hell (as they get more computationally expensive to generate, electricity consumption soars). This essay has some questionable numbers, but the underlying principle is sound.
Bitcoin mining software is now being distributed as malware because using someone else's computer to mine BitCoins is easier than buying a farm of your own mining hardware.
Bitcoin violates Gresham's law: Stolen electricity will drive out honest mining. (So the greatest benefits accrue to the most ruthless criminals.)
Bitcoin's utter lack of regulation permits really hideous markets to emerge, in commodities like assassination (and drugs and child pornography).
It's also inherently damaging to the fabric of civil society. You think our wonderful investment bankers aren't paying their fair share of taxes? Bitcoin is pretty much designed for tax evasion. Moreover, The Gini coefficient of the Bitcoin economy is ghastly, and getting worse, to an extent that makes a sub-Saharan African kleptocracy look like a socialist utopia, and the "if this goes on" linear extrapolations imply that BtC will badly damage stable governance, not to mention redistributive taxation systems and social security/pension nets if its value continues to soar (as it seems designed to do due to its deflationary properties).