These two cameras are said to use the same sensor, so I compared converted RAW files at a variety of ISOs using this test scene:
Throwing in a Panasonic G1 for good measure, here are the ISO 800 crops processed in Raw Developer with no sharpening or NR:
Overall, my feeling is that the GRD III hangs tough with the S90. The GRD shows slightly less noise but also slightly less detail. Having printed high ISO files from both cameras, I'd say they are equally capable for shutter speed-limited, low light work.