Two quick remarks. First, there already are many virtual/digital currencies, some of which simply peg their currencies to the dollar. But nobody really cares. Why? Well, what's the point? It's just a slightly less convenient form of a dollar.

Second, we all already do use digital currency. Actual, physical currency (and deposits at the Fed) are called the monetary base or "high powered money". However, most people transact goods using debit cards, which is called "inside money". These are privately created dollars, and they change hands digitally. So, we're already there. (I'm avoiding credit cards since those are technically loans, not money).