This article isn't really about map-reduce, but anyway.
Map-Reduce on GPUs
It depends whether your code is suited to GPUs. If you're map-reducing a bunch of single precision floating point data it may be a good idea.
Still not sure if Java is a good front end to OpenCL.
It doesn't matter IMO; you're still writing the code that does the actual work in OpenCL. Think of it like a different flavor of JNI.
