Well, it only takes some simple maths...
Most modern PalmOS handhelds (PalmOS 5) or even PocketPC's are in the region 200-400MHz. Now admittedly, most of them are ARM processors, the same as the GP2X but any sort of emulation layer is going to bring in a performance hit (assume 50% in even the average cases - think about the SNES, which has only a 20MHz processor and runs full-speed on the 200MHz GP2X (just about), providing you don't need any other chips emulated. So most things that run on PalmOS just aren't going to run on the GP2X.
Now, the older Palms go down to silly numbers that are theoretically emulatable but there you are talking about ancient software that just isn't worth the effort to emulate. And there's not a lot of source code for emulators for any of the above (notice the stickies at the top of this forum - WE MUST HAVE SOURCE CODE TO PORT TO THE GP2X - that means writing it ourselves or getting it from somewhere - nobody is going to write a Palm emulator from scratch for you).
No source code = no chance of a port. From previous searches I've done, there were zero open source Palm emulators for anything vaguely modern. Even the official Palm "simulator" is a pig to set up and it doesn't come with any source code. People need source code to port things - you might have seen Doom and Quake and all sorts on the GP2X but that's either because (a) the source code for the game, or something which can run the game data directly, is available or (B) the source code for an emulator (e.g. DosBox) that can run the original game without changes is available. Neither are true for Palm/PocketPC.
And even if it was possible, I can't think of any "killer" games that I'd like from my Palm on my GP2X, plus the screen sizes are completely incompatible, the processor isn't even anywhere close to practical for emulating a Palm, plus almost any game would need Palm ROM's and all sorts and Palm are a lot more protective of their ROM's than most companies.
It just won't work. Next time, read the stickies - SOURCE CODE. Find a PalmOS emulator with source code and (if someone could be bothered) someone could you how impractical the idea is.