Thought I'd post this for LED LCD HDTV buyer's out there thinking about purchasing their next TV. The quotes in this thread are from an LED article from Gizmodo.com.
I strongly suggest reading Gizmodo's article: Giz Explains: What's So Great About LED-Backlit LCDs - Led - Gizmodo before buying your new TV. It's very easy to understand if you're not into techno jargon.
There are two major kinds of LED backlighting: Edge-lit and local dimming. Edge-lit displays are what they sound like—the LEDs are arranged in strips running along all four edges of the TV, like you can see in this gut shot from Cnet. A light guide directs the glowyness toward the center of the screen. The advantage of edge-lit displays is that they can get incredibly thin, are 40 percent more power-efficient than regular LCDs and are a bit cheaper than local-dimming TVs. But because they're still shooting light indiscriminately across the LCD panel, they can't pull off the black levels that a local dimming backlight setup can
LED backlighting of the local dimming variety is how you build the best LCD TV in the world. It's called local dimming, as you probably guessed, because there are a bunch of LED bulbs—hundreds in the Sony XBR8—arranged in a grid behind the screen. They can all be dark or brightly lit, or they can turn off individually or in clusters, making for the actual Dark Knight, rather than the Grayish Knight you'd see on many cheaper CCFL LCDs. Sets with local dimming are pricier than edge-lit—the Samsung's local-dimming 46-incher started at $3,500, versus $2800 for one of their edge-lit models. They are thicker too.