I think the limitations that RISC OS has on desktops are just as problematic on portable devices.
One of the reasons Psion eventually died is because of the lack of a good Internet experience, poor interoperability, no toys like decent audio or video playback etc.
Portable devices these days already do much more than RISC OS desktops can. My Nokia N810 can:
* Use an iPod as external harddisk
* Play high quality video from DivX or MP4 files.
* Poke my friends through Facebook and other Web 2.0 apps with the Firefox browser
* Go online through a wireless access point or Bluetooth mobile phone - where are the drivers and stacks for RISC OS to enable this?
* Use "native" YouTube through its Flash plugin
* Run Java applications (almost) through the in-progress Jalimo
* Run NetSurf natively
* Run ArcEm
It's just as programmable and hackable as RISC OS (if not more so) etc; runs on a 400MHz ARM processor with 128MB of RAM and has advanced power management.
RISC OS would be a cool distraction on such a device but it'd be useless for getting any real work done, and you'd lose so many of the features and gain very little. The expectations of a mobile device already exceed what RISC OS can do.
There is *no* consumer market for RISC OS, it's too old, too archaic and is supported as a hobby. It should be viewed - for most people - not as an investment, but as a hobby as much as a Hornby train set.