Currently no flashcard can access the DSi features or run DSi games, access the SD slot, DSiWare, access the cameras, etc. The problem is, the DS is hard coded so that when you place in a normal DS game (Well, non encrypted and digitally signed DSi game) which includes flashcarts, the games run under a DS compatibility mode which will either need a hardware mod or custom firmware to bypass which restricts the DS game to only what is available on a normal DS. This means that normal DS games and flashcarts will not be able to access the new DSi extra features.
Now a custom firmware is currently not possible until some one manages to break the encryption and digital signatures that the Nintendo uses for their firmware, DSiWare and DSi games. The firmware on the Nintendo DS was only protected by hardware which could be easily bypassed by using a paper clip which allows the firmware to be over written. With the DSi, the firmware is updateable via WiFi as well as through DSi games though it is digitally signed and encrypted with a very strong encryption which will not be easy to crack.