natrium42, the original PassMe designer, has released a guide for building a serial interface on the Nintendo DS. It does require you to flash a new firmware on the Passme, but other than that it is just a matter of soldering wires to the RX and TX. He’s tested it with a keyboard, a bluetooth module, and a USB adapter. If you want to hook it to a computer you’ll need the level converter he suggests. There is a lot of potential here.
In other news: StoneCypher has offered a bounty for an open source TCP/IP implementation on the DS; it’s already broken $1100.