I remember readint this somewhere , but I don't know if it is true, but apparently the Plasma's BBC LDN have burned on to the screen, but it wouldn't suprise, unless if they switch them off, but you never know.
But it can happen on any ordinary TV Screen or PC Monitor, I read on 625.uk.com last year about some guy who bought a new Widescreen TV and his kids watched CBeebies for a couple of hours a day and after a short while the CBeebies DOG had burned onto his screen.
It too wouldn't suprise if the screens at Cartoon Network aren't damaged as they have had the same DOG 24/7/365 for the past 8 Years at least, and it has never moved or changed either.