Wintermute has been kind enough to open source the DSi hack we have been using. To run the hack you will need a way to read and write save files on DS carts.
Tutorial:
1) Download the hack for your DSi enhanced game from
http://github.com/WinterMute/
2) Edit the asm source file to reflect your region (US and UK currently supported), and of course add your own custom code.
3) Make the project which should produce a .sav file
4) Load the save file onto the game cart.
5) Launch the game in your DSi
Currently you are limited in space which restricts you to assembly programming. Hopefully this will inspire a few more people to poke about and get info coming in on SD card access and, of course, the cameras.
Loading a save file:
I have been using a tool called eepinator to upload the save file using a regular DS. There are probably other tools which would work.
http://www.dcemu.co.uk/vbulletin/showthread.php?t=57752