DXO reports that both Dynamic Range and Noise go down by the expected (linear) amounts all the way to 100 ISO, so I think it's fair to consider that "Native." Though considering the way that ISO seems to work in these cameras, I don't think it is right to think of there being "native" and "non-native" ISO settings. There are examples of some cameras where this seems to be the case (5D MKII has worse performance at certain intervals) but not with most, including the GH1.
From my testing I think there is both analog and digital gain going on at *all* ISO's. The only way to truly minimize this is to shoot in UniWB (ie everything is green).
I asked because someone told me that the native ISO levels for the 5D MKII and 7D are 160, 320, 640, 1250 & 2500. Everything else is just exposure compensated.
