Is that due to some kind of issue with the architecture, or just a matter of software support?

In the latter case, I'd expect patches for AMD or Intel to become a priority pretty quickly. After all, they need their products to run on systems that customers can buy.

Well, they don't support CUDA and I don't see CUDA coming to AMD any time soon.

Intel is just plain not capable of it because it's not really a GPU, more a framebuffer with a clever blitter.

Great opportunity to adopt Khronos APIs.