Region Free (DSi Unable to play out-of-region downloads)
Non-DSi games (physical cards)
First off, if you buy a physical DS game, it is 100% region free – any DS card will work in any DS. If this is all you intend to do, have fun – you won’t run into a single problem.
DSi downloaded titles
The problem with the DS lies with the new DSi – namely, it’s downloadable titles. As the handheld has no multi-account system, there is currently no way to have multiple accounts on the system with different regions – and the handheld is locked to whichever region it was purchased in. To play foreign titles, you’ll need a new DSi for each region, sadly – as well as a way to buy the points from that area.