HPCWire - there are 35 TOP500 systems with NVIDIA GPUs (twice as many as in June). Of these, three of the top five supercomputers are equipped with GPUs, with more on the way in 2012 with the 20-petaflop Titan system at Oak Ridge National Lab and the 11.5 petaflop Blue Waters super at NCSA.
In Huang's SC11 keynote, he pointed out that the rise of HPC-style GPU computing has come about because traditional CPUs, especially x86 ones, have become rather inefficient at compute- and data-intensive computation. For example, he said CPUs use 50 times the energy to schedule the instructions and 20 times the energy to move the data than doing the actual calculation.
GPUs, by contrast, are designed to reduce data movement, and although they have poor single threaded performance because of their simple processing engines, there are many more of them to do the work in parallel. That makes for more efficient computation, assuming the application can be molded into the GPU computing model.