I notice that DXOMark has just published its measurements on the new Canon G15. DXOMark's review did not explicitly compare the G15 with the G12, so I used the compare cameras function to do so. You can see the results below.
What I find particularly interesting is that Canon changed sensors from CCD (G12) to CMOS (G15). The G15 does not have the (Candle symbol) Low Light Mode that the G12 has. The G12's low light mode uses pixel binning to boost the ISO, while dropping resolution to 2.5 megs and improving signal to noise ratio.
The G12 has pixel pitch of about 2 micrometers. Binning the pixels combines signal from four pixels so they behave as one, effectively producing a pixel pitch of 8 micrometers (which is better than any of the new full frame slrs, in terms of pixel pitch). I think that is a good thing for signal-to-noise, at the sacrifice of resolution. I note that if you press the FUNC SET button while in Low Light Mode, it brings up a menu for setting the ISO up to 12,800.
Lately, I have really been enjoying the low-light capabilities of my G12. Here's a sample, taken with the Low Light Mode just last night at ISO 400, f4, 1/160 sec, 71.7 mm equivalent focal length.