I was not around in 2010, but from what I read on the forums, the 1st client that was made public was a CUDA client released by user puddinpop(bitcointalk) after Jeff Garzik donated 10k BTC to him.
But I think the reason OpenCL is used by Laszlo is because he was a Mac fanboi(I think he is also 1st to offer binaries for mac), and OpenCL was created by Apple.