PalmOS 5 looks pretty interesting. The OS itself is written for ARM architecture, but apps are still compiled for m68k. So even though they get interpreted, the performance hit isn't so bad since most apps spend their time waiting on the OS.
For actual processing, there will be an option in the new compilers to make native ARM chunks for algorithms, so that when processing needs to be done and ARM is available, it gets used.
I'd be interested to see how well it works in practice. Of course, I haven't even touched a Palm device with > OS 3.5 on it, so that probably won't happen any time soon :P