The way I read, the Oly is only a concept and the Panasonic is a reduced sensor 18mm x 13.5mm, using the 4/3's format. It can be used with other 4/3 lenses via an adapter ring, but may/may not be compatible with it's autofocus. It has no reflex mirror and images have to be composed using the LCD. Here are the conclusions from DP Review
for the Panasonic G1.