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.
The dividends are paid every 6 months and the ADR issuer will announce the date just like a normal dividend (may be slightly later than the Japanese share date). The dividend in December was about 86 cents/share, but this can obviously vary quite a bit based on profits and exchange rates - like many foreign companies, Nintendo targets a percentage of yen-denominated profits rather than just paying a fixed amount each quarter.
The ADR issuer will withhold and pay 8% foreign tax on your behalf. In an IRA this money is a loss because there's no corresponding US tax to cancel out (i.e. that part of your IRA income is now foreign-taxable and the IRS doesn't care because it's an IRA). In a standard account you can take a credit against your taxes for foreign tax paid.
Either way you do ok, because this is a pretty low withholding level -Europeans other than UK often take out 35%. UK is just about the only major country that doesn't withhold taxes on dividends to US holders. The ADR issuer will also charge you a small fee, around 2-3 cents/share, in exchange for passing through the dividend in dollars. Otherwise I think there is no fee on the ADR (they don't take anything off the purchase/sale of shares or charge a holding fee). Overall, holding Japanese ADRs definitely beats owning a mutual fund with excessive expenses.