to the question of true DLC for DS, yeah, i guess that's highly unlikely... unless they opt for something like what the
Art Academy cartridge did, and make a whopping 8MB save file. if they managed to up save file sizes from 512KB to 8MB (x16, i think), then wouldn't it be possible to just UP the save file size further? Of course, then they'd have to figure out what's the best limit to cap it off, and the actual cart size to use...
that's all just wishful thinking though.
on the DSi however, maybe they can find a way for a game to have FULL access to the DSi's system memory and SD card? that potentially provides the game with unlimited storage, although of course, access speed would probably suffer, but still... that would be TRUE DLC.
somebody should really make a proof of concept for this (for DS; DSi mode will have to wait till it's cracked wide open).
EDIT: this is awfully gorgeous. will hold off from trying this until one can actually start to do something in it. i just hope the DS's controls can control this baby just fine