Regarding this Shader Model 5.0 (HLSL) talk, the greatest reason (in my opinion) for Nintendo to utilize something equivalent would be if they could get compute shaders running under GLSL or whatever API they are using. While compute shaders (also known as DirectCompute) are heavily bound to Microsoft, AMD and NVIDIA have worked much with it and I do not believe it's impossible for AMD to provide an API that can mimic what HLSL 5.0 does with compute shaders, or they could in conjunction with the Khronos Group supply OpenGL under a certain designation.
Here's a direct comparison of using compute shaders on Direct3D 10 and Direct3D 11 hardware:
Source:
http://msdn.microsoft.com/en-us/library/windows/desktop/ff476331(v=vs.85).aspx
Now, the amount of threads available when using compute shaders on Direct3D 11 hardware is easy to see and why Nintendo would use, but more interestingly, atomic instructions are available under this mode. I'm not even going to try to explain atomic instructions as it more or less requires several sheets of paper, but here's a common definition:
Easily put, linearizability is more expansive and easier to use than general sequential consistency, and linearizability also provides slightly lower development costs over its "competitor". If I were a developer creating Wii U software, I'd be delighted to have something like this to work with (as well as POR and a good overview of possible primitive types). Other than this and some buffers, HLSL 5 doesn't excite me too much though of course I see the immediate advantages over HLSL 4.1. I can see Nintendo using a HLSL 5 equivalent if they want to future proof Wii U, but if they are just aiming for a current gen compliant offering they would surely go with 4.1. But I think seeing as Epic has been vouching for Unreal Engine 4 lately and it's more or less inevitable success, the first option is more likely, if not for effects and such then for administrative work and development time.
Here's also a presentation by AMD regarding this (from SIGGRAPH '08):
http://s08.idav.ucdavis.edu/boyd-dx11-compute-shader.pdf