One thing at a time, I think. You're correct that each platform will eventually need a coherent host-side opencl subsystem for heterogeneous device management, but I expect there will be competing implementations, at least for the first while.

As for saving driver writers effort; I suspect the host-opencl->device interface will be a lot like current graphics drivers: very dependent on the platform architecture for graphics at the higher level, and very dependent on how the hardware works at the low level. OpenCL itself should help by making a lot of mid-level driver code more portable though.