I'm going to start from a selfish point of view, because I own a Palm.
I think including tests and flashcards and possibly even the jlpt listening material in some basic interface that could be used on Palm OS would be great.
From a more broad perspective, possibly a mobile java based thing rather than palm os specifically.
Ipods are good to an extent, but the screen is a bit insufficient and theres no proper interactivity like you can get on mobile devices (especialy Palm pilots or Pocket PCs).