Joystiq have reported that a small group of homebrew developers (Tobias "0xtob" Weyand and Collin "TheRain" Meyer) in Germany are currently developed a system dubbed "DS Midi WiFi" that allows the DS to exchange MIDI data with a PC.

"The exchange works both ways, so users can tap out tunes on their touch screens (using an accompanying keyboard app), while PCs receive and output the data; or, more sophisticated tracks can be composed using a MIDI sequencer on PC, and then beamed to DS for playback."

Hmm, interesting. Check out the video from YouTube.