The comments here are a lot more sane than all the other discussions I've seen. It's still not obvious to me that the IRS will regard transaction outputs as discrete objects instead of fungible units like shares of stock (where you can use rules like FIFO).
If HN has already discussed this particular point to death, my apologies, I missed it. But I don't understand how Bitcoin can achieve its apparent aspirations if my tax consequences are different depending on which bitcoins I spend.