Does anyone know if it is possible to run RSView ME on a Windows Mobile PDA or similar? If not, any ideas what kind of HMI package can?

RA doesn't provide an installer or license mover that would allow RSView ME to run on a Windows CE target platform, so the simple answer is no with regard to RSView ME.

Windows CE is a lot more version and hardware dependent than the 32-bit Windows families are. You need to run different compilers to run on different CPUs (ARM, MIPS, x86, etc) and the version differences (CE 4.1, 5.0, 6.0, and Mobile 4, 5, and 6). I don't know of any major industrial vendor who is taking the open-hardware approach and none that are using Windows Mobile.

I've seen some small software like Palm-based Gnarly Software's simple HMI.

The approach I have seen is users running Windows Terminal Services Client on a CE or Mobile box, and then running a Terminal Server session of RSView SE Client. Usually they are just adding a set of displays to a bigger system, like adding fuel-loading truck displays to a co-gen power plant visualization system.