Go here for the Olympus E-M5 images, and go here for the Panasonic GH3 images. You can do your own comparison by opening up the images into new tabs/windows.
Obviously, these images tell us little about the dynamic range and full color capability of the sensor at various ISOs, but they do tell us how the camera will perform at just spitting out completed images. Overall, it is a big upgrade from the GH2. It's still something of a disappointment. Noise performance closely matches the E-M5 up to ISO3200, but ISO6400 shows a significant drop in comparison to the E-M5.
This could still be the same Sony sensor, and perhaps Panny's JPEGs just suck, but I doubt it. The difference at ISO 25,600 is so large that it simply has to be a different sensor. This might not be seen as a huge issue, but with the Olympus E-PL5 having even better performance than the E-M5, that puts Panasonic at a large disadvantage, especially going forward.
I remain excited about the GH3. ISO performance is only part of the equation, as we all learned well with the Fuji X Pro 1. That said, this is a disheartening moment. I might be more willing to overlook it if the video performance had been truly outstanding, but everything we have seen thus far has been, at best, whelming.