Ok, I checked on the page you said, and you are right the pixels are 1280 x 768. However my model is the PDP-505PU but is the same. So I stand corrected on this. But why does my TV display show 1080i on the display? When I connect some games it shows 720 and boy what a difference in quality.
I wrote the post you quoted at the same time you wrote about reading your manual so I was adding to the conversation and not correcting you. However, thanks for being understanding that I am just trying to help you.
SHORT ANSWER
All it means is that your set is decoding a 1080i signal.
LONG ANSWER
Understanding one's digital television has always been confusing. Manufacturers have taken advantage of the newness of their products.
Most people are confused by two concepts. Manufactures have made it appear that they are saying the same thing.
1. Monitor can play 1080p, 1080i, 720p, 480p, and 480i.
2. Monitor has 1080p resolution.
Most people think that the first statement means the second statement is true.
Reality is that the first statement ONLY says that the monitor can decode those signals. It doesn't say what resolution it plays those signals at.
My bedroom set is like your plasma. It can decode 1080i but it plays it in 720p resolution. Just like your plasma, my set says it has a 1080i being decoded.
My main set is a 1080p that plays in 1080p. I waited almost two years to buy it. When 1080p first hit the market, few sets marked 1080p had 1080p resolution and those sets were out of my price range. I bought when the prices came down.
Many people who bought those early sets believe that they have a real 1080p set when all they have is a set that can decode 1080p.
BTW, my first HD set was a high quality native 1080i rear projector. Your 720p plasma has a far superior picture than that set. So while resolution is important, it isn't everything.
PS
I sold my first HD set to my brother-in-law cheap. His kids still use it in their basement and it still has a good picture. There are ways to recycle an out of date functioning set.