I can give you a quick list of apps/etc the Pixi cannot currently handle-

VisualBoy Advance, SNES Emulator - both use arm7 code, Pixi has arm6 cpu. Anything labeled ARM7 is for Pre only, ARM6 for Pixi only

Any flashlight LED patch/app - Code isn't accessible yet

Safe overclocking kernels - There's some great Pixi kernels, but only one has overclocking, and I wouldn't recommend it

Wifi Mobile Hotspot - WILL work if you have Pixi Plus with Wifi, but not original 'Sprint' Pixi

That's really all I can think of... there's a few more things though... Most 3d AppCat games work very well on the Pixi, try out Angry Birds Lite! Most of the time when people say something is for the Pre they mean WebOS in general, and any of the devices that are out. Usually!