I went down to CC today to check out the Samsung LN-S4092D, and much to my delight they had it setup right next to the Sony Bravia KDL-V40XBR1, which until now was easily the best looking LCD I had seen. They were showing a recorded Giants-Dodgers from ESPN HD game off an HD TiVo connected via component video. I didn't have time to ask them to switch over to a DVD, but since I watch a lot of sports, the broadcast they were showing was fine with me.
First, let me say that both sets looked excellent, or at least they did after some tweaking from the ridiculous defaults. They both had good black levels, vivid colors, and attractive bezels. The Sony displayed noticeably more detail in shadows, but I thought the Samsung had a more dynamic, three-dimensional look. Each set had its strong point on colors -- the Sony had better reds, but the Samsung had better greens. And the image on the Sony looked slightly overcast compared to the Samsung, though I'm not sure which was more accurate. While neither showed any significant degredation at normal positions, the viewing angles on the Samsung were slightly better than the Sony. Overall, the Sony probably had slightly better picture quality, but it was very, very close. More tweaking, possibly with the service menus, could definitely close the gap somewhat.
I won't recap the specs here since everyone probably knows them, but personally I'd rather have 2 HDMI inputs than 1 HDMI plus a CableCard slot. I should mention that the Sony required much less adjustment than the Samsung, though this isn't a big deal when you actually own the television. For movie watching, I would think the Sony has a bit of an edge due to better detail in dark areas, but I think the Samsung would be just as good for sports or other bright content. Ultimately, the $500 premium in MSRP for the Sony doesn't seem worth it after seeing both sets side-by-side, and I think I'm going to get the Samsung.