PostmanPot
Honorary Master
Well, you will in some cases still get image retention, but it is not permanent... If you watch a movie with black bars (for example a movie with 2.35:1 aspect ratio) you will probably see faint lines where the bars were for a few minutes on a full screen image afterwards. Occasionally it is worse, and people have image retention for a few hours, but it is relatively uncommon.
In general, with modern plasmas you don't really need to worry about it.
IR is very dependent on how high your brightness/back-light is set to.