Yes. It's the same tax treatment that applies to gold bullion or barrels of oil - volatile commodities that are at least partly speculative investments rather than stable mediums of exchange.

What is the tax treatment that applies to national currencies?

Country-specific, but often you don't have to account for them up to a certain limit, and/or you can treat anything you buy in a foreign national currency as having been bought at what you originally paid for that currency.

Not sure in the US, but in Canada you don't need to report capital gain on foreign currencies if its under $200.