I think it's a grey area, but I am not an accountant. The government considers it to be intagible property in some scenarios and a capital asset in others. For the most part I think the capital asset happens on the conversion to fiat due to the realizing of gains and losses. If anyone else has more knowledge here, I would love to know for certain.
I'm 100% certain that absent a ruling letter the current guidance from the IRS is that a transfer of one token for another token (e.g. Ethereum for Bitcoin via Poloniex) is considered a taxable event, similar to, for example, selling gold and purchasing copper. I'm not sure if Ethereum tokens would be able to stretch into a different definition, but I suppose time will tell.
https://www.irs.gov/uac/newsroom/irs-virtual-currency-guidan...