Andrew: sorry, I should've been more explicit. It's possible to put together a completely open source software stack machine and sell it *without paying any royalties*.
Doing the same with ROOL's shared source would, as you say, only be possible when paying Castle a royalty.
However, it's interesting you say "entirely possible to build a box using open RISC OS components and sell it." Is it the intention, then, that ROOL will eventually share enough to create a bootable system entirely from source?
RISC OS 6, free for end-users, running on RiscPCs is an appealing prospect, given the RPC still makes up the majority of current hardware.