In many ways the PDA has a limited number of interface possibilities available to it for music, and yet in other ways so much is possible.
Many applications use interfaces that are very similar to laptop / desktop applications, but are re-designed for a significantly smaller screen. Other applications go in a different direction and actually exploit the nature of the PDA in their interface, although this is rare.
Here’s what it looks like:
But I think that the PDA or handheld can support many other types of interface as well. The Tenori-ON concept could (I think) be applied to the PDA interface. There’s a very good article all about the device itself and background on Create Digital Music which gives lots and lots of information about the device and specs.
But I don’t think that the possibilities really end there either. Perhaps I’m too hopeful or too enthusiastic about the subject, but I think there’s lots more life left in mobile music.