As many here are most likely aware, the IRA recently released the following guidance regarding virtual currencies. Here is the link:http://www.irs.gov/uac/Newsroom/IRS-Virtual-Currency-Guidance
The IRA will now categorize virtual currencies as property and require US users of the 'currency' to track and report capital gains and losses incurred on each transaction. This ruling destroys the fungible nature of crypto coins because now each coin is different. Each coin now has a cost basis that must be taken into account when the coin is used. Spending coin A may result in a taxable capital gain of $300 while another may result in a capital loss of $32. It will all depend on the value of the coins when they were obtained compared to the value of the coins when they were spent.
I believe ether will all fall into the IRA's 'property' category and therefore suffer from these same fungibility issues. However, I think the problems are amplified with ether and Ethereum due to ether being used to power the contracts in the blockchain. US residents are either going to ignore their ether's cost basis when they decide to pay the fees for a contract, or they will have to be very diligent in their record keeping.