Bitcoin has been mentioned for the first time in the US Supreme court decision 'Wisconsin Central Ltd. v. United States' on the the Railroad Retirement Tax Act of 1937 (archived). In a decision which determined that stock options are not cash compensation, Stephen Breyer dissented and attempted to weasel that stock options are equivalent to cash with:

Does a stock option received by an employee (along with, say, a paycheck) count as a “form”—some form, “any form”—of “money remuneration?” The railroads, as the majority notes, believe they can find the answer to this question by engaging in (and winning) a war of 1930’s dictionaries. I am less sanguine. True, some of those dictionaries say that “money” primarily refers to currency or promissory documents used as “a medium of exchange.” See ante, at 2–3. But even this definition has its ambiguities. A railroad employee cannot use her paycheck as a “medium of exchange.” She cannot hand it over to a cashier at the grocery store; she must first deposit it. The same is true of stock, which must be converted into cash and deposited in the employee’s account before she can enjoy its monetary value. Moreover, what we view as money has changed over time. Cowrie shells once were such a medium but no longer are, see J. Weatherford, The History of Money 24 (1997); our currency originally included gold coins and bullion, but, after 1934, gold could not be used as a medium of exchange, see Gold Reserve Act of 1934, ch. 6, §2, 48 Stat. 337; perhaps one day employees will be paid in Bitcoin or some other type of cryptocurrency, see F. Martin, Money: The Unauthorized Biography— From Coinage to Cryptocurrencies 275–278 (1st Vintage Books ed. 2015). Nothing in the statute suggests the meaning of this provision should be trapped in a monetary time warp, forever limited to those forms of money commonly used in the 1930’s.

The single mention of the word 'Bitcoin' has generated substantial headlines, but the real story here is how far Breyer is willing to go in an effort to help the criminal gang in Washington DC pick a few more pockets.