On openshading the FAQ says:
OSL only works on the CPU for now. It would be possible to port the system to GPU, but it would require a lot of work. There are no immediate plans for it.
What does this mean? Does this affect only OSL script nodes? I noticed that a a OSL script wave_rot.osl is compiled in wave_rot.oso which looks like assembly source code (but not for ia64 so I guess it is code for CUDA).