Rob that is a great question and one that has come up many times before. I even asked the Pioneer representative at CES this year the same question. His response was that it was a PC resolution that has been standardized. He was of course talking about XGA which is 1024x768 but it is the same premise. More pixels is better, there is no arguing that, but the question still remains where do the rest of those pixels come from and how can they make a 16x9 display from a 4x3 resolution? The answer is actually pretty simple; it is not a 1:1 pixel mapping and pixels don't have to be square. In fact the Pioneer Elite plasma (arguably the best plasma) has a 1024x768 resolution and rectangular pixels.
I know what you are thinking now; How can it look as good if they are scaling the 1280x720 or 1920x1080 video to a different resolution? (Well that is what you should be thinking.) The answer is surprising or it at least is surprised me. All TVs scale, yes all! The single biggest difference between a TV and a computer monitors is that TVs have overscan. The reason isn't a good one, but that is the way it has always been and that is the way it is now. That means that if you buy a TV with a native resolution of 1280x720 and the TV has ~3% overscan, the circuitry in the TV is throwing away 3% of your pixels and scaling the rest to fit into the 1280x720 pixels on the display. There are some TVs that have the ability to turn this off, they call it "computer" mode others call it a 1:1 pixel map. The problem with theis modes is that TV production folks expect you to have overscan, so they use the outside pixels for things such as Closed Captions or Neilson ratings, it is refereed to in the industry as the vertical blanking interval. It also happens to be how those MovieBeam movies get beamed.
The bottom line is it really doesn't matter how many pixels your TV has as long as it looks good. If you are like me and your have an OCD for these type of things, buy a 1280x720 TV that you know has the ability to produce a 1:1 pixel map and ignore the garbage on the vertical blanking interval. If you are not brave enough to try it on your own, then there are ISF calibrated technicians who can do it for you.