I too have the same plasma (50pz850) and noticed it retains images 'image retention' very easily also. ie in ps3 menus, it happens very fast, only takes about 30secs, maybe even less? the retention is visible but then dissapears very fast, gets wiped out when somethign different is displayed. . I've had the plasma since early december, so im pretty sure ive used more than 200 hours already. However, my brother was playing GTA4 recently, for maybe about 6 hours straight, and some more the night before, I noticed the bottom left map and health bar, retained a white circle. Its the longest image that has been retained so far, after several hours (about 5 at least), the retention has started to fade away. I've also had to disable the map/health bars too.
Also, turning off the tv at night, whilst the room's lights are off, I notice alot of image retention, shown as ghostly bits of things on the tv screen, making the black screen seem like it has dirty patches of white on it. They aren't noticable the day after when you watch tv again though.
I wonder how much image retention can it take, before it actually 'burns in'?