Plasmas dim an all white screen for power/energy reasons since that is when they use the most power on white/bright scenes. LCDs work the opposite with more energy used to block the back light in dark/black scenes. In a dark room an all white screen would burn your eyes out so this has never been an issue for me with my plasma.
That being said, you could be seeing some of the issue because of your settings also, so getting a calibration disc is advisable too.
