This post from TUAW seems to have found some evidence.
Another iPod mixer from Alesis. iLounge have the story. Not really handheld though.
kBang I picked this up from CDM. Jowan Sebastian has built a brilliant, elegant app for Game Boys called kBANG.
Wire up a Game Boy to other objects — like solenoids, simple mechanical devices that can perform a tapping action — and you’ve got a real-world drum machine. Enough glitchy beats: physical objects become percussion.
Here’s what Clickgamer says:
It just like a small electric piano. No matter where you
are, you can use Pocket Piano to play a song very easily.
You can also use this software to compose a song.
The features of Pocket Piano Pro:
Support ad-lib mode, quiz mode and compose mode
Support 3 octaves
Support 36 keys
Support 3 leves of listensing comprehension quiz
Support metronome function
The beats count of metronome is adjustable
The BPM (beats per minute) of metronome is adjustable.
The metronome click could be disable.
Support editing functions (copy, paste and delete) in
Playback speed (tempo) of each song is adjustable
Support play repeated function”