On the Tokyo stock exchange, the Japanese have to buy in 100 share blocks, so it is a ~$28,000 minimum investment. Citigroup maintains the US pinksheets NTDOY.PK shares and 8 of them = 1 share on the Tokyo exchange. You can buy however many you want as it is an electronic representation of the paper shares Citi holds (I think).
Sometime in the future (June 2009?) Nintendo will have to start using electronic shares on the Tokyo exchange. They may do a stock split and they may maintain their own ADR on the NYSE, but I haven't seen anything concrete.
Unfortunately all I know has been skimmed from various articles and I can't say that I understand how dividends are paid and what Citi gets for hosting the NTDOY.PK shares.