I'm also curious about this. Let's say you have an altcoin like ETH. You trade it for BTC, and then later you sell the BTC for $. Do you pay the taxes at the point where you did the ETH -> BTC trade (and hence that is your new base), or do you pay only when you do the BTC -> $ sale (using the original base you may have paid for ETH)?
So no answers to this question?