I don't, but I do spend a lot of time on other forums where most members do - I don't think it's a problem unless you played the same game for about 2 weeks and there was an icon on the screen in a fixed position. I'm sure some people have done though...
The only time I have seen screen burn on a plasma was in Richer Sounds where the black bars had burnt into the demo screens showing films on the wall(ALL THE TIME!!)
Panasonic TH37-PX70, Cambridge 540r v2 & 840a amps, Cambridge Audio 640c CD, Toshiba HD-E1, Humax PVR-8000T, Sony HD-NW5, B&W CM1s, Mordaunt-Short centre/rears, REL sub