The good folks at DPreview have published their long awaited review of the G12. Although titled a "Quick Review" because of similarities between the G12 and G11, this is still quite a comprehensive review. Even so, DPreview recommends reading the G11 tests and review as well.
I was very surprised to read that the sensor is the same as the sensor in the G11; in fact, I doubt this is the case because the (Sony supplied) G11 sensor was widely reported to be incapable of supplying data at the rate necessary for HD video. Perhaps the G12 sensor is a slight variation of the G11 sensor. Also noted is that Nikon's P7000 uses the same sensor.
Well worth reading.