mental images' RealityServer has now the option to render on a GPU cluster with CUDA based iray.
A very impressive interactive ray tracing demo, first shown publically at Nvidia's GPU Tech Conference: http://www.megaupload.com/?d=RCYDVG6S (H.264, 180 MB)
A video of iray integrated in Google SketchUp: http://www.youtube.com/watch?v=i6LBhws8l-A
RealityServer is sold in different configurations consisting of 8 up to 100+ Tesla GPU's. The biggest cluster configurations are targeted towards rendering of online virtual worlds and online entertainment, and can serve multiple concurrent users per GPU.
more info:
http://www.hpcwire.com/features/NVIDIA-Pitches-GPU-Computing-in-the-Cloud-65217572.html
http://brightsideofnews.com/news/2009/10/20/nvidia-launches-gpu-based-realityserver-30.aspx?pageid=0
Nvidia also showed a demo of an interactively raytraced Bugatti with global illumination running on OptiX at GTC:
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video: http://www.megaupload.com/?d=YAOSYWK1 (H.264, 82 MB)