It can be any region of the retail game new or used. But if you buy it digitally it has to be the Japanese version,
This will also work on firmware versions from 4.0 upto 9.2
HTH
It has to match the region of your 3DS though.
Yup, it's definitely rising fast on Amazon! Earlier today before it was announced, you could get a used copy of Cubic Ninja for $2 before shipping. Now the lowest I can see at a glance is ~ $16. What fun!
Glad I bought Cubic Ninja when I did then.