I am pretty sure selling into "digital fiat" on an exchange is a taxable event.
But what if you're just day trading it to build your investment? That could get messy trying to figure out each gain and loss every time you buy and resell even a small portion. And to my knowledge, none of the exchanges send out 1099 forms to tell you your gains/losses, do they?
...but yea, you're probably right. Knowing the IRS, they probably would consider fiat transactions on an exchange taxable; but they'd have no real way of knowing unless you tell them. But be that as it may, I'd still think BitUSD is not the same as cashing into USD, and thus you'd just be making a trade of one investment for another with no gain or loss to recognize (like-kind exchange, basis in the new asset = basis in the old asset). If BitUSD were to start being accepted as
real USD, however......that would be sweet. And probably taxable.