From the 700 series Zaurus onwards, there has been sufficient processor power and memory to allow emulation of other platforms. Of course, it is quicker to provide a virtual machine/application library interface than to emulate an alien platform where the processor and peripherals are totally different from the Zaurus's.
The most exciting development as of writing this is the Zaurus port of the PSX emulator. http://www.oesf.org/forums/index.php?showtopic=17052
Other emulators exist:
Palm Emulator copilot is useable but not particularly quick. Maybe when PalmOs 6 comes out, which is linux based, there will be progress
NES - x86 - Bochs and QEMU
-- unfinished, but hopefully experts will add stuff.