When I only had 2 GPU WUs I noticed that one was on the slow GPU rather than both being on the fast GPUs. The Event log reports:
20/02/2012 21:40:50 | | NVIDIA GPU 0: GeForce GTX 460 (driver version 28562, CUDA version 4010, compute capability 2.1, 1024MB, 684 GFLOPS peak)
20/02/2012 21:40:50 | | NVIDIA GPU 1: GeForce GT 430 (driver version 28562, CUDA version 4010, compute capability 2.1, 512MB, 179 GFLOPS peak)
20/02/2012 21:40:50 | | NVIDIA GPU 2: GeForce GTX 460 (driver version 28562, CUDA version 4010, compute capability 2.1, 1024MB, 684 GFLOPS peak)
20/02/2012 21:40:50 | | NVIDIA GPU 3: GeForce GT 520 (driver version 28562, CUDA version 4010, compute capability 2.1, 512MB, 104 GFLOPS peak)