I've only shot with a friend's m43 for now, but I've been interested in this system for a while. I think the current Panasonic 16mp sensor is quite good. If the RAW from E-M5 are significantly better for high iso and dynamic range then it must be very close to my D5100/X100. I could very well join in then, particularily with the lenses coming this year (oly 75mm 1.8).
Chroma noise in dark areas looks so different that I am sure it can't be one of panasonic's sensors, and this is what they are officially saying now.
Noise "grain" dimension is probably not different from G3's one, but what changes is detail and shadow tonal range. Light areas are not the problem: G3's sensor is on par with other good cameras. It's in the medium/dark areas that you are going to lose... everything. Apparently (looking at these raws), the new sensor is very good in the dark areas. Perhaps the raws are pre-cooked, we will know later, but those darks at 6400iso look very very clean and detailed, and I don't see the typical color smearing of a heavy chroma noise reduction. Of course, more examples are needed.
complete image for reference. 6400iso - RPP (see the uncorrected barrel distortion...) -