>>266315563DS can't emulate DS games.
What it can do, however, is run them natively. However, for cartridge games to be run natively, they need to be converted/recoded into DSiWare. DSiWare has a specific file limit, plus rather than running off the SD card, the 3DS can only run them off of the system memory which is severely limited (I think it's like 256MB).
So it's not really feasible for DS games to be downloadable on the 3DS at this time.
