from my experience (I have a US-NTSC XB360 in EU)
for physical games I import everything from the US it follows the regular DVD zonage you have on films....
now for digital... it is dependant on your XBL account you can have a US account on a EU or japan Xbox in which case you have to go either on the US xbox.com website to purchase and add to your queue or trick your console and MS XBL to think you are in the US with a VPN
so technically on any XB360 you could have an XBL account from any region but for it to work properly you need 2 things one to geo relocate your console in that country through a VPN and to have a credit card in that country or use pre paid cards or maybe (not sure with that one) a paypal account in that country....
if you are American I would not bother and get a US XBL account... your physical games from japan will work and your digital games bought from US XBL should work... at least it does for me I have 2 XBL account from 2 different continent and respective content from each zone work fine....
now just to be save maybe create a 1 month XBL account just to check if it does work properly that way.... but for me it works that way digital content work with corresponding XBL account location...
