Reports from those testing flash carts on DSi suggest Nintendo have blocked access to them by denying auto boot.
When the revised DS model, the DSi, was announced back at the company's fall conference some had speculated that Nintendo would take action into flash carts and homebrew on their portable baby. According to the GBA Temp forums it's looking worrying for those who do own one of many different types of cart as they now seem to be blocked from access.
For those that do load up onto the DSi's revamped menu, an error appears that prevents the console from accessing the card, but for the majority the card fails to show entirely. Whether or not homebrew folk can override the change is currently uncertain, but given past the performance and mind-numbing speed of the talented guys behind the carts, it shouldn't be too long before DSi owners can run their software once more.
Carts that have failed include: CycloDS, G6 DS Real, M3 DS Real, Supercard DS One, iTouch DS, FCard, M3 DS Simply, NCard and U2DS.