From wikipedia
"The Nintendo DS is in the sense that any console will run a Nintendo DS game purchased anywhere in the world; however, the Chinese version games can only be played on the Chinese iQue DS, whose larger firmware chip contains the required glyph images. Although Nintendo DS of other regions cannot play the Chinese games, iQue DS can play games of other regions. Also, as with Game Boy games, some games that require both players to have a Nintendo DS game card for multiplayer play will not necessarily work together if the games are from different regions (e.g. a Japanese Nintendo DS game may not work with a North American Nintendo DS game, though some titles, such as Mario Kart DS, are mutually compatible). With the addition of the , certain games can be played over the Internet with users of a different region game."