While there's OpenCL drivers available for Linux, on Ubuntu there is no OpenCL driver shipped by default and the proprietary driver implementations aren't always great.
Intel has their closed-source OpenCL SDK for the CPU, AMD has their OpenCL support in the Catalyst driver, and NVIDIA has their OpenCL/CUDA support bundled within their binary driver. When it comes to open-source support, the Gallium3D OpenCL support is still primitive and isn't shipped by default. Intel has the Beignet project
for open-source OpenCL but that's even worse off than the Gallium3D OpenCL support.
For details on the experiences of using OpenCL on Ubuntu 13.04 and other information, an open-source developer, Michal Hruby, has written about it on his blog