I can't comment on the technical issues with a port, but maybe with some of the smaller vendors like Cathena, they'd be willing to help out and provide specs for video, sound, network hardware etc. Also with the Nokia 810, it runs Linux, so maybe the source for drivers is available (I've not checked, just guessing).
There are a lot of problems to overcome, but I think it would be a very exciting time for RISC OS to have people who would buy a RISC OS device for reasons other than hardcore fanatasism. I really think that RISC OS has a shot as an Ultraportable Operating System, it won't get to be mainstream or anything, but it would mean that you could sit someone in front of a device running WinCE, and one running RISC OS, ask them which they prefer, and maybe get a lot of them preferring RISC OS.
For everyone considering an EeePC or Nokia 810, we can say "Have you considered RISC OS?", we'd start off with computer enthusiasts like OSNews readers and go from there.