I've put together a collection of about 300 puzzles as small images which can be displayed on palm devices capable of displaying images:
http://www.net-chess.com/puzzles.zip
These are optimized for display on the iPod nano, but since they're just JPG files, they'll work on pretty much anything. It's really at the "proof of concept" stage right now. All of the puzzles are white to play, and no solutions are given. And since there are no solutions, all of the puzzles are either mate in 1, or just a move or two deep as far as tactics go.
Here's a few samples: