I was perusing http://www.khronos.org/ web site and only found headers for OpenCL (not OpenGL which I don't care about). How can I obtain OpenCL SDK?
AMD's ATI Stream SDK works perfectly for me and it uses multicore cpu's. I have an Intel CPU and an NVIDIA card but it works with using the CPU. Just registration is required and no special selection like Nvidia requires: http://developer.amd.com/GPU/ATISTREAMSDKBETAPROGRAM/Pages/default.aspx
I got it to work in ubuntu 9.04. Just download the installation instruction PDF:s also available on that page and it should work.