For the 42" models from the likes of Pioneer, Panasonic, LG and others, the resolution of 1024X768 utilises rectangular pixels to give a 16:9 ratio.
The reason for this is due to the historical purpose of plasma display panels, which were developed as computer monitors prior to their adoption by the consumer market. Hence they use VESA (i.e. PC) resolutions such as SVGA (800X600) and XGA (1024X768).
50" models from the likes of Pansonic (with a NR of 1366X768, or WXGA) use square pixels - as that resolution is naturally 16:9.