I think for the money we're spending on these sets, we should not have to "settle" for lines on the screen. These tvs, including the ST50, are supposed to be amongst the best of the best. The ST50 is in a very small group of plasmas that are considered best of the year, along with the GT50, VT50, and Samsung's E7000 + E8000 sets.
Overall I'm blown away by the image quality of the set. I really am. But the line is a bit troubling because it's not the type of thing you would associate with such quality televisions. Of course neither are green blobs (Panasonic 2011), fluctuating brightness (Samsung 2011/2012), etc. - it's a shame each set has to have their own set of issues. They're all so close to being amazing, but they fall short with some silly issues.
I still think the ST50 is an amazing set. The image is so stable. Not a hint yet of fluctuating brightness within scenes. That alone puts Panasonic at the top this year as far as I'm concerned. I hope I'll hear some info on the line from Panasonic.