Larry White has a great write-up of the Free Competition in Currency Act of 2011. It’s important that he notes that it would not eliminate the USD, or even the Federal Reserve’s role in currency markets. Rather, it would re-create the open competition which existed for over a century in the United States. The best part about this act is that it does not mandate anything either way. If people decide that they want to use only Federal Reserve USD, they are free to. But if there are better alternatives, people are free to switch.
One problem that I could see is that the elimination of the USD as applicable to “all debts” is that there could be higher transaction costs in negotiating currencies. This would also require that the Supreme Court not try to re-establish Juilliard v. Greenman.