Actualy, it is true, but the ratings have grown over the last few years on Plasmas to not worry about it any more. I think the 9th gen Pannys are rated at 60,000 hours which is 8h/day for 20 years.
This is for last years models:
"The product life span specification increased to 60,000 hours half life (half of the plasma panels initial level of brightness) in normal conditions. Panasonic has changed the panel structure to a new bound cell design (a technique adapted from Pioneer technology). This structural change improves light emitting intensity and efficiency thus allowing for brightness improvement. Products life span is also extended as a result of the lower power requirement to effectively light cells."
