http://www.geek.com/hwswrev/psion/goby/
for the psion

and for the palm OS operated PDA's
the liberty gameboy emulator, it will cost you though about 16 bucks.
LIBERTY GB EMULATOR

LIBTERY GAMEBOY EMULATOR REVIEW

A 100% free GB EMULATOR FOR PALM
PHOINIX

To use these, you must have the Gameboy Rom image. I found about 600 of these on edonkey. They work flawlessly, aside from the sound problems when playing a few games.
BB