I had a brief play with ittyMIDI a long time ago when I had a Sony clie. The nice thing about the clie was that it had an on board midi chip so you instant access to GM sounds. The down side was that Sony had not implemented the Palm Sound API and so you needed a third party hack to run things like Bhajis Loops.
Anyway, this is what the ittyMIDI site has to say about the software.
ittyMIDI Player converts your Palm OS® compatible hand held into a full featured MIDI player and will transform the way you practice or perform. To see for yourself, download the free demo today. ittyMIDI Player works on many Palm OS devices. Please check the compatibility list to see whether your model applies.
Most handhelds require an external synthesizer to generate polyphonic MIDI, although a few models have built-in synthesizers. Again, please see the compatibility list for details.
Sony Internal Synthesizer Support
Palm OS 5 Support
MIDI OUT to External Synth
MIDI THRU from External Controller
Works with Swivel SG20 and Tsunamidi synths
Works with Handiclip
Count In Option
Disable Alarms Option
Program (instrument) Mapping
GM to non-GM Mapping