A lot like OpenCL
'River Trail' provides things like ParallelArray which has a method that can execute a user-given function in data-parallel (ie: running the function for each element in the ParallelArray, passing the function the index in the ParallelArray as well as any other params that the user initially passed through). So effectively SIMD and quite a bit like invoking on GPU kernels in OpenCL - in fact, Intel even says something to this effect.
Still, it doesn't make me look forward to coding in parallel under JavaScript any more than threads would do. But then Google's Native Client isn't any more innovative either - just different problems, but headaches nonetheless. I wonder if we are witnessing the beginnings of the fracturing of Web programming?