Palm OS 5 was the first to support ARM devices. It'd probably be faster to emulate that, because you could probably directly execute code on the processor. I'm not even sure if that's considered emulating anymore.
EDIT: Perhaps not. The only Palm 5 emulator is closed-source (it's also made or licensed by Palm Inc), but there's open-source Xcopilot which emulates the monotone Palms using the Motorola processors.