Well, it seems that the OSX running iPod Touch could be a good candidate according to this post on the Bhajis Loops forum.
“It seems that the new nanos and the new classic still run the old software stack.
However, the new iPod Touch (= iPhone without the phone feature and the camera) runs the same software stack as the iPhone. I assume that in a week, some hacks will be found to run third party apps on it.
Once I’ll be able to afford one, I’ll buy one and see how easy it is to develop audio apps on it. I am not familiar with the iPhone UI toolkit, but it really sounds like a strong candidate platform to develop audio apps.”
PalmSounds 