Playing Devils advocate here. Where do you get the value of bitcoin at time your trade is made? Since bitcoin can vary by a hundred or more
That is a problem for which I'm pretty sure there's no official IRS answer. I downloaded daily Bitcoin prices (I think from Quandl) and used those. Since the IRS doesn't provide guidance for this I would hope that I'm ok as long as I use a consistent source and don't cherrypick the Bitcoin values.
If it ever gets to this point, you'll be getting 1099's from your exchange of choice, and there will be no more trading in the US without a taxpayer ID. When you see that, then it's time to worry, or maybe they'll have figured out how it should be reported by then. Until that time, try to report it as you see fit, and you should be fine.
The IRS isn't really a big bad machine. I had issues in the past, I was able to make my case, and almost took it to tax court to get it documented, but instead settled it. It wasn't as scary as you would think, and eventually they saw the error of their ways...
This might be getting a little off topic for the Monero spec thread, but had to see what others are doing. Thanks for the input.