People are still talking about plasma burn-in, even though it's a non-issue. Actually, two very rare separate non-issues, permanent burn-in and temporary image retention. Gary at HD Guru decided to put the matter to rest:
Gary decided the best way to illustrate this issue is to figure out what it takes to create image retention or burn-in.
In his trials, he was able to cause retention, "a faint ghost image of previously viewed content," by tuning to a SMPTE colour bar test pattern for 15 minutes and then switching to an all white screen. The resulting effect dissipated after a few minutes and is definitely of no concern to plasma display owners.
Burn-in, a "faint outline of a previously viewed image caused by uneven phosphor wear," turned out to be far more difficult to cause. Gary couldn't do it by freezing a frame for ten hours, nor by seeking out showroom demo plasma HDTVs that might have been on a burn-causing loop. In the end he concluded that the effect might be created in a "worst case scenario" which involves "100% viewing of 4:3 content with black side bars [instead of gray] , or exclusive viewing of 2.35:1 aspect ratio movies (without using one of the zoom modes to eliminate the black bars) and leaving it on that way continuously for weeks."
So basically, unless you're seriously obsessed with a particular image that you'd stare at it for weeks at a time, or only watch 2.35:1 movies (in which case who cares if the burn is uneven in the unused pixels?), you can skip worrying about burn-in and enjoy your plasma. If you want more on the science of all of this, check out Gary's full story. [HD Guru]