Technically yes, the same rules apply as if you were trading in any international currency. If you sell yen at a profit to convert to USD & purchase something, you owe tax. What sort of tax can be complicated depending on how long the asset was held. I'm not an accountant, but I think assets held less than a year are taxed as personal income, so marginal rates apply. Assets held longer than a year are, I believe, taxed as regular capital gains.
I'm not sure what would/should happen with extremely short term trades, i.e, you convert to Euros from USD for a vacation, the USD crashes, you convert back upon return from your vacation. I doubt the IRS worries too much about it, but I suppose technically you should file Form 8949 along with a Schedule D.