the original DS is not region locked- can play any game
the DSi however, has a region lock for the DSiWare downloadable games, as well as DSi-specific cartridges. But it can still run any DS game from any region
the 3DS is locked for DSiWare, and Nintendo DSi-enhanced software between Japan, America and Europe/Australia
so in simple words, it is region locked if its a DSi/3DS only game