ATI Stream SDK 2.0 was the first public implementation of OpenCL available to anyone! This new release is still limited to CPUs and support is not complete but that's a great platform to learn OpenCL which actually means sens to me as well.
ATI Stream SDK 2.0 beta 3 is available for Windows XP/Vista/Seven through Visual Studio 2008 and Linux through GCC 4.3 and ICC 11.
I notice on the beta 3 FAQ, some really interesting information:
"The GPU version of OpenCL to be bundled with the ATI Stream SDK is scheduled to be available later in Q3 of 2009" ... "The first production release of the ATI Stream SDK v2.0 with OpenCL 1.0 support is scheduled to be available in Q4 of 2009."
I expect to see a public release for nVidia soon as an implementation for MacOS X Snow Leopard is available and apparantly working pretty well. ATI implementation for MacOS X is... almost unexistant.
The Khronos Group has opened (ugly PHPBB) forums for OpenCL, I hope to hang as soon as possible!