That's a very nice HDTV, btw ;). A certain amount of IR is normal w/ a plasma. Now if it didn't go away quickly like you've described, then you have a problem per se. FWIW, there's some excellent discussion of this particular phenomenon over @ avs forum-
I bought a Panasonic 50" plasma this fall (model TH50PZ850U). After displaying shows/games with static images I often see a ghost image of after switching inputs to display a black screen. This goes away after a minute or two of moving images. Sometimes the ghost image is of something that was only displayed for a few seconds. Is this normal, or am I not taking proper care of my new HDTV? I have about 120 hours on the TV.