I guess the strict definition of open source is about, well, open source. However, the dual licensing thing just adds another dimension - it doesn't take an existing dimension away. That something is available under a closed source licence doesn't mean that the same thing, licensed under an open source licence, is suddenly not open source for all those people who have received the software under the terms of such a licence. Indeed, commercial agreements often run in parallel to the availability of software as open source and do not compromise that in any way, and I'd be surprised to see any open source definition expanded to cover what is effectively a business strategy.
Anyway, the "shared source" licensing of RISC OS "Open" seems like an odd combination of GPL-like distribution terms with a commercial exploitation restriction. It'll be interesting to see what effect that restriction has, and whether the GPL won't ultimately prove to be a better choice.