The Korg DS-10 is a cartridge for the Nintendo DS that turns the portable game device into a synthesized MS-10 synthesizer, including two synth engines, a drum machine, and a 16-step sequencer. You can punch in controls with three different interfaces, including an on-screen keyboard. And while you can't link multiple DS-10s through MIDI or USB, the DS-10 does support controlling multiple DS running the software through a wireless interface.
It's Japan-only, but since the DS doesn't use a region lock on its cartridges, importing the 4,800-yen title should be no problem. In fact, I might try to grab a couple myself after the July release—they're sure to be a collector's item.
It's worth noting that this isn't an unofficial project; Korg actually sanctioned AQ Interactive to make the software.
Product Page [AQI.co.jp via Music Thing]