IIRC, there has been only one major upgrade in the m4/3rds NMOS sensors (I will leave the GH-1's sensor out). There has been tremendous progress in the AA/UV/IR cut filters that are in front of the sensor, both in size and in performance. Astounding piece of engineering there.
The very reason that Oly and Pana are sticking with the m4/3rds NMOS sensors (after Kodak terminated sensors to Oly) is the cost. NMOS sensors are cheap. The ones in the camera are on a flex board unlike ceramic cased CCD or CMOS sensors (of any size), for example. I still can't believe that this is the piece that does the image recording.