This guide is aimed at UK/EU games buyers:-
With Christmas approaching and to avoid disappointment, it's important to understand the main difference between the older Nintendo DS Lite consoles and the new Nintendo DSi ones.
The most important fact is that Nintendo DSi consoles are 'region-locked', which means that they will ONLY play DS games from the same region.
For example, if you purchase a UK DSi console, then only DS games manufactured for sale in the UK/EU will work on that console. Previously, for the DS Lite, you could purchase a game from anywhere in the world, knowing it would work on your console, but this is no longer the case. So when looking to buy DS games for your DSi console, games bought from outside the UK/EU will NOT work on UK systems.
Whilst you cannot always be sure, a good practice would be to buy only from UK/EU sellers, furthermore check the packshot, as UK/EU games are different to ones purchased in the US or Hong Kong for example. UK games either show the BBFC (U, PG, 12, 15, 18) game classifications or the PEGI ones (3+ 7+ 12+ 16+ 18+), whereas none UK/EU games show the American ESRB rating system (E, T, M, R etc). Some sellers in the UK/EU sell imported games from the US or Hong Kong, which will NOT work, unless your DSi was manufactured for sale there, so check first before you buy!