After realizing that you can run the DSi pictochat .app on a DS by converting it to nds (aka renaming), I wondered: Does any DSiWare run in DS-mode?
Nintendo Fan Network supports WPA/WPA2 connections, which only a DSi can use, so, probably not. Pictochat DSi (with the rainbow pen and whatnot) and DS Download Play run in DS-Mode, I guess those are it.Anything that requires a save would have to be compiled with the twl sdk to read off of NAND.
So I don't think there are many chances for ds mode apps. Maybe Nintendo Fan Network? lol