Actually you might be able to get away with it. Japanese electronics run on 100V while America is 120V.
Generally, this difference is inconsequential, as most appliances are built to tolerate current a certain percentage above or below the rated voltage.
If you are using an american Cube in Japan it should not be a problem, the other way around (japanese Cube in America) should still be OK from what I have heard.
My friend bought a mini-MP3 player in Akihabara and asked about this same thing, the answer he got was "it should be ok but long term use may damage extremely sensitive electronics".
The AC adapter is built to take the damage of incorrect Voltage, I made the mistake of plugging a Japanese Cube into a Thai outlet (220V!!) and it fried and burn the Adapter but the Cube turned out to be fine after I got a replacement.