I started work a few years ago on making a cube map able to be blurred by some arbitrary amount in real time. The first 5 methods got close but always failed.. then I took a year off from CG or so.
I finally had a realization of what I could do to make the cube blur work, and, one computer upgrade, and some system crashes later. I have it done.
It suffers the same sampling artifacts as the 2d version did, and I'm quite uncertain how to remove them, However, it would seem with much of any normal map contribution, or on a less shiny material, they may fade to be obscure enough to not be an issue.
The prime benefit to this is you can use this technique with realtime-sampled cube maps, not just pre-rendered ones.
If you are using pre-rendered exclusively, it should be far more efficient to just do the normal thing of lerping between a diffuse convoluted map and a sharp reflection map. More memory and a texture sampler needed would be needed for that, but a far more simple shader setup.
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Edit: Derp. Nvm. In the udk sub forum, didn't even notice...:)
Also here's the shader network for the blurring.
Other things this opens up is a better version of imperfect refracting materials, something sorta like realtime captured image based lighting.
The single bit there is I believe 109-115.