The G1 X handles high-ISO-sensitivity shots with aplomb.
The G1 X does produce very good JPEGs up through ISO 1600, which is excellent for this class of camera. Though decent raw-processing software isn't yet available, given Canon's history I'd say that you probably won't be able to eke better performance out of the raw versions, just different trade-offs.
Even ISO 3200 JPEG shots can be quite usable. You can see a little color noise (look at the whites of his eyes) and clipping in the shadows, but the edges and color look pretty good and there's none of that mushy watercolor effect produced by overcompression in the luminance channel.
(1/30 sec, F2.8, ISO 3200, spot metering, AWB, 28mm)
There's a lot of factors that go into high-ISO-sensitivity image quality. Here, I suspect that the X100 delivers better results not just because of the size of the sensor--it's bigger--but because it has a brighter, faster lens.
The cheaper Olympus E-PL3 displays more color noise in its JPEGs than the G1 X. But the beauty of the ILC is that you can always buy a better lens and improve the results a bit; you can't do that with the G1 X.
This was shot using the G1 X's Handheld NightScene mode, which combines 3 shots for a reasonably exposed night shot. On one hand, it's kind of mushy; on the other, it's equivalent to ISO 12800. One thing to watch out for is motion; those blobs in the lower right of the photo are a couple of people walking through the scene.
(1/20 sec, F5.6, ISO 12800, evaluative metering, AWB, approx 99mm)
You can choose from three monochrome color schemes, black and white, cool (cyanotype), and sepia. You can't control the contrast, though, and I wasn't thrilled with the odd infrared-look of the background.