the Contrast ratios for Plasma tvs are all marketing. Here is a general rule of thumb when shopping around for flat panels.
Contrast (higher = truer Blacks and truer Whites)
LCD < LED-LCD < Plasma
Plasma TVs have the best blacks because each "cell" or pixel is lit or unlit individually and give off light themselves. An LCD on the other hand has a back light that shines through the LCD cells to produce color resulting in non-uniform brightness and non-true blacks. And LED-LCD is the compromise. It places LED cells behind LCD cells. This way the LEDs can be turned on or off in areas unlike a LCD with a back light where the back light is always on.