Honestly, writing decent PS3 apps is not that hard for any decently skilled computer scientist. Lots of middleware supports it very well these days -- perhaps that's why Valve doesn't want to jump in... they don't use middleware, as I recall, or perhaps their own physics, etc. systems just don't go parallel/stream so well, as they would need to do on the on the SPUs.
Valve may just prefer to keep a very slim development staff, and therefore maximize profits in the short term. As I recall, some company (Google?) was wanting to purchase Valve recently for Steam -- they may want to keep their dev budgets low until that blows over.
Or maybe they have some wierd aversion to cool parallel architectures that no-one but they can understand. Who knows. Honestly, I can't think of many engineers who don't find working on cool new architectures fun. I can think of plenty of business folks who dislike it, though.
