Depends on the target architecture. Some GPUs have used vectors in the past, and some people try to run shaders on CPU. (like for OpenCL or for emulation)
Which ones? I'm aware of AMD/ATi Terascale using VLIW, but I'm pretty sure that architecture also used SIMD (requiring the use of both VLIW and large 'waves' to achieve maximum occupancy).
And running shaders on CPU is, in essence, a similar programming model to ISPC. When you run OpenCL on a CPU I am quite certain that the runtime pretends each lane is a program instance, same as ISPC.
