I just returned from another BB visit and spend more time with the 85HU8550 as well as the 65X950B, 79X900B and the 65HU9000. I brought along my Spears & Munsil disc, we also tried The Dark Knight blu-ray on most of them (expect the 900B), the third Narnia film in 3D (only only the 8550), a live DirecTV feed and the demo reels for each set.
I don't feel I definitely answered the question. But I am going to vote edge-lit. The thing is there were no obvious issues that made that assessment a slam dunk which speaks to the quality of the panel I saw even if edge-lit. The uniformity was truly great. There was no clouding or flashlighting at all. It was only when we put on a grey screen and a check board of grey and white that I could discern a very, very small amount of light bleed from the lower right quadrant. It was subtle. Not something you'd see in normal use at all.
The black level was also okay but not truly great. Not in the same league as the 950B and a bit worse than the HU9000 but not a lot. It may have been on-par with the 79X900B, which doesn't match the black level of the smaller screens in that model.
The most tale tell sign of edge lighting I saw was when I ran the motion test from the S&M disc that scrolls their logo repeatedly across the screen. On the 950B it showed no discernible halo or blooming at all. On the HU9000 is had a bit of a halo but it was pretty localized. On the 8550 the entire space to either side of the logo was a tad lighter than above or below it, from screen edge to screen edge.
On the other hand, as I said, it took a lot of work to find an obvious sign of edge-lighting. In normal programming, playing The Dark Knight, I couldn't tell at all. In fact, with the HU9000 if I viewed off-angle I could see a bit of lighter shaded vertical banding about a third of the way across the set whereas I didn't notice any problem off angle with the 8550. This could have been an effect of the curve in the screen, but it seemed like I was seeing some light bleed with the HU9000.
It is possible, I suppose, that if Samsung slapped a FALD panel with their standard 8550 firmware and chips that it might show some of the imperfections I noted. But my gut is on edge-lit. Very, very good edge-lit. Enough that I am now adding this set to my short-list.