A friend of mine used to compose music using MIDI software that ran on the PalmPilot. He said riding around on the bus was a fairly inspiring way to write songs. Frankly, I'm surprised there aren't more portables designed for making music, rather than just listening to it.
Thanks to FastTracker 2-style app called NitroTracker that runs on the Nintendo DS, now anyone with one of those can sit on the bus seeing what it was like to compose music on a computer in the early 90s using early "tracker" software for the PC. Tracker software combines aspects of MIDI and samplers to let you manipulate samples, notes, effects, tracks, patterns, and more to create fairly robust sounding songs.
If you don't have samples, NitroTracker can grab samples from the Nintendo DS's microphone. One could even figure out how to create a whole song live, out of nothing more than one's voice, if one were so inclined.
