The Tech Report writes NVIDIA has finished the first OpenCL 1.0 conformant drivers for Windows XP and Linux. It's not available for everyone though:
This "conformant" driver isn't downloadable by just anyone, mind you. Interested parties must first sign up as part of Nvidia's GPU Computing registered developers program. Still, the driver itself purportedly supports all CUDA-enabled graphics processors, which should cover pretty much all GeForce 8 and later offerings (not to mention more esoteric products like the Tesla series).