Wow that was fast. NEOflash is the first flash card for the Nintendo DS.
Details are sketchy, but at this point it’s rumored to use a special bootcart in the DS slot and a second card in the GBA slot that can be used to store either GBA or DS games.
UPDATE: Photos are now posted at the site above. NEOflash appears to be a product of the XGFlash people, complete with the SlimLoader and problematic removable cart battery. This is probably a product worth avoiding for that reason alone, especially with alternate DS firmware like this in development.