DavidPilling: "I agree, with RISC OS on top of a unix kernel it could be a serious contender. Or to put it more correctly without doing that it is not a serious contender."
Wise words indeed.
AMS: "would Unix do RISC OS well ? I don't think so."
Mac OS X does classic Mac nicely enough, so there's a notable precedent that you're ignoring.
sa110: "Why all this talk about porting RO to another platform. One of the main problem,in my opinion, is the lack of software development in general."
The two things aren't entirely independent. Having the machine lock solid because some kernel module (erm, relocatable module) dips into the OS workspace or some hardware register really undermines any attempt at a reasonable development environment.
simo: "I'm not sure what made RISC OS great anymore, a few killer apps from ComputerConcepts (now surpassed by Scribus, Xara etc.)"
I'm not vouching for Scribus, and Draw still does some things more elegantly than Inkscape, although that's mainly due to the inability of developers to maintain usability whilst adding features, but there isn't any part of the RISC OS experience I can think of that isn't surpassed by something like KDE plus GNU/Linux. Back in 1994, RISC OS could have taught the UNIX desktop scene a few tricks; now the influence is definitively reversed.
JGZimmerle: "This basically means, that if an identifiable section of source code also works without the GPLed parts, then it is considered a "separate work" and the GPL does not automatically apply to it."
This is why large parts of KDE, including derived works based on KDE code, can be distributed under the LGPL, although a complete, functional KDE distribution would most likely be licensed under the GPL due to the underlying library licences.