There are no surprises in the G11's noise profile. Photos look clean at ISO 80 and ISO 100, while softening begins at ISO 200. At ISO 400 you can begin to see some degradation in detail, in addition to the softness, and by ISO 800 there's enough blue-channel noise to produce some yellow splotches.
ISO 400 was a bit disappointing for this class of camera, especially given the drop in resolution. There's still sufficient detail for a lot of scenes, but you also see the yellow blotches and some white pixels from the noise suppression.(1/20 sec, f2.8, spot metering, AWB, ISO 400, 28mm equivalent)
Depending upon scene content, ISO 800 shots may be usable scaled down a bit. Switching to raw at high-ISO sensitivities didn't help much (processing using either Adobe Camera Raw or Canon's Digital Photo Pro); the JPEGs are fairly well optimized. (1/20 sec, f3.2, spot metering, AWB, ISO 800, 28mm equivalent)
Thanks to the drop in resolution, the G11's photos display significantly less noise than the G10's. The trad-eoff is that at ISO 80 and ISO 100, where both cameras are relatively noise-free, you lose the extra resolution the G10 provided. Overall, though, the G11 delivers more-accurate colors as well.