I've had Samsung, LG, and Pioneer plasmas and I've used Panasonic plasmas a lot as well. Genuine "burn in" really isn't an issue so far as my experience goes. I've played videogames for hours and hours with a static, white HUD on all of them and non have ever had permanent burn in.
I HAVE spotted non-permanent image retention on all of them though. And while I'm not nearly anal enough to recommend a full on "break in" period where you don't watch any content and do nothing but run patterns and solid colors for several hundred hours, I WILL say that pretty much all of the image retention issues occur when the panel is new.
Plasmas do seem to "settle" after a few hundred hours of use. When I first got my Pioneer Kuro, I played videogames with HUDs and saw some image retention. Scared the crap out of me!
Especially since it took quite a long time to fully disappear using the "wipe" pattern and just watching regular TV.
Now though, with several hundred hours logged, I can play the same videogames on my Kuro and it doesn't show a trace of image retention. So it was definitely showing up when the panel was new, but now that the panel has "settled", it doesn't seem to be a problem at all. I'll tell ya though, I was scared at first! But yeah, it was never permanent, it wasn't "burn in". It was just non-permanent image retention and now it doesn't happen at all after several hundred hours of use.
So if you spot image retention of those black bars from 2.35:1 movies, don't panic
In my experience, it's totally normal. After the plasma panel has "settled", those "ghost" images will fade away into non-existence. Run the "wipe" pattern for a few hours if it's bugging you. But don't freak out. Eventually, your panel will "settle", just like all of mine did