I think I have found my answer, so in case anyone is interested:
yes, the new sensor in the GH2 is actually read at 1080p50 or 1080p60 for its 50i/60i output modes, apparently with half the lines of each frame then deleted to produce the lower data rate 1080i50 or 1080i60 that the rest of the camera can handle. Most of that answer comes from a spec. page that I overlooked before:http://panasonic.net/avc/lumix/systemcamera/gms/gh2/specifications.html
and the rest from discussions of samples available on youtube and vimeo.
I suppose that motion should look smoother than with the "60i from 30p" method.