In reply to Mr Chocky: if you have any pointers to stuff written on the subject I'd be interested to read them. Aside from the trite "it's only software so all it takes is work" aspect there is the example of Apple moving from ancient CMT on 68k to a full unixoid on PPC (and doing so extremely well) as motivatin. Sure, we lack a few hundred million dollars but what the hell.
I don't imagine you'd disagree that the UI could be supported. I have tried ROX a few times and although it has diverged rather a lott it showed the principle quite well.
There is also the example of VMWorks (I think - the app that lets you run unix under windows, windows under unix and so on) as an extreme way to tackle it. THey also avoid the different cpu problem.
Perhaps rather than saying 'run RISC OS on top of a unix kernel' it might be more useful to say "could an OS be built on top of the kernel of linux/bsd/whatever that would be sufficiently like RISC OS to please users and attractive enough to developer to get them to reowrk their software".
Sadly I lack the large amount of spare cash to offer to support such an effort. Wish it were otherwise.