I was in the local electrical store yesterday comparing these two sets.
To my amazement, they looked almost identical in terms of picture quality, maybe the plasma edging it in terms of the movement (tested on Sky Sports News) and easily had deeper blacks, in fact much deeper. The demo disc being used was the Panasonic one with the stupid dancing fruit things.
During this demo, there is a screen which is entirely black except from the word Panasonic in the middle in white. This is when you could see the huge difference in the black levels. I had tried messing about with the settings, but couldn't get it anywhere near the plasma.
By the way, as a note of caution, I checked the cables etc being used to input the signal into the box (sad, but I always do). The PX80 had an HDMI and the LCD had compnent. I was absolutely baffled, why have a 1080p screen on display and only use component. I asked the salesman, who seemed none the wiser, before I walked out in disgust at his lack of interest.