The one thing that worries me about RISC OS (and I'm sure this would have been one of the reasons many people left the platform, before the release of the Iyonix/A9) is whether there will be any further development of both the OS and the hardware to run it on. If open sourcing RO makes it more likely that Castle and others will be able to get OEM work (as suggested by Peter Wild) then it would presumably be a positive development.
But whether or not open sourcing drives more development, from the desktop perspective it does at least mean that the OS cannot disappear suddenly if the owning company (like Castle or ROL) disappear, as almost happened with Acorn. Moreover, it opens the possibility for RO to be made to work on any of the many interesting ARM devices that are already out there, which would be great.
So I'm with flibble on this. Open sourcing won't necessarily solve all of RO's problems, but there may not be a great deal to lose. The only possible danger is that there might be massive branching, and maybe this could be controlled by Castle/ROL? I think it could well be a very positive thing from the users' perspective.