Yes, 42" plasmas are well known for not having a single display that uses square pixels in a 16:9 format at full HD resolutions. You have 3 typical layouts for pixels in a plasma...
853x480 - true square pixels, true 16:9 format, not HD - called EDTV
1024x1024 - Now how does that come close to anything? Very rectangular pixels - but more than 720 tall so it can be called HD.
1024x768 - The most common 'HD' layout for a 42" plasma - uses rectangular pixels.
Yes, feeding it a 1024x768 computer image will fill the 42" plasma completely and will stretch everything out. Most plasmas I believe allow you to force it to show as a 4:3 image in the center, but it has to throw out some of the information to do this, so you lose some image quality.
