Here's what's in the LX428 manual:
Information of pixel defect
Plasma television displays information using pixels. Pioneer plasma television panels contain a very large number of pixels. (Depending on the panel size: over 3.1 million cells for a 50 inch/60
inch display). All Pioneer display panels are manufactured using a very high level of ultra-precision technology and undergo individual quality control. In rare cases, some pixels can be permanently switched off, or on,
resulting in either a black or coloured pixel permanently fixed on the screen. This effect is common to all plasma televisions because it is a consequence of the technology. If the defective pixels are visible at a normal viewing distance of between 2.5 metres and 3.5 metres whilst viewing a normal broadcast (i.e. not a test card, still image or single colour display) please contact the supplying dealer. If, however, they can only be seen close up or during single colour displays then this is considered normal for this technology.