In April of this year, Tyler and Cameron Winklevoss revealed that they owned about $10 million worth of Bitcoins, or about 1 percent of all the Bitcoin wealth in existence.
At the time, Bitcoins were in the middle of a sickening crash, tumbling from an all-time peak of $266 per bitcoin to $105 in the course of one trading day. The price has continued slowly downward, recently valued at about $90.
Unlike other currencies, Bitcoins are not issued by a bank or other centralized authority. Since 2009, people have been able to generate or "mine" Bitcoins by participating in the global computer network that supports them.
The ETF would let traders and investors in on Bitcoins -- without having to go through online currency exchanges, which are notoriously difficult to access.
The currency remains anonymous, unregulated and untaxed. The Bitcoin ETF, however, has fees and taxes.
Despite an 18-page outline of the "risk factors" involved in buying an ETF tracking Bitcoins, and numerous articles enumerating the risks, the Winklevoss twins think people will want to let them take a cut on bitcoin investment transactions.
At any time, Bitcoins -- which are just digital -- could be erased from existence, through computer error or hackers. At any time, governments could decide to regulate, tax, or even ban the currency, which happens to be useful to money launderers.
And, as with any ETF, there is the risk the fund won't match the value of its underlying assets.
Some think the Winklevoss IPO will legitimize the digital currency, but critics argue ETF investors will have to take on all the risks of Bitcoin without any of the benefits of being untaxed and anonymous.