I work at a pretty large electronics store and have known about the Samsung buzz since last year and was going to pick up a PN58B850 as they were going discontinued but figured Samsung surely would be able to fix the buzzing issues as I saw a countless number of returns for that exact reason.
Once we got the PN58C6400's in I was quick to put one on demo as I was waiting for them for 3 months and right away I could hear a slight buzz when right up to the screen but it mostly sounded as if it was coming from the back and I couldn't really hear and substantial buzz. So I went ahead and bought one knowing if I got a buzzer I could easily return it.
After getting it all wall mounted and set up I sat down and was blasted by buzz, I was extremely disappointed that Samsung hadn't fixed it and upon further investigation it is shocking how directional the buzz is. If your sitting anywhere off the outside edge of the bezel you cant hear anything, if your sitting even slightly off center 8-10ft back its not bad but once you get right in the middle it will give you a headache after a few minutes, its just as loud as a xbox and trying to watch movies is terrible as any time there is a quite scene or dialog the buzzing is overwhelming
I then went back to our demo unit to see if that one did have as bad of a directional buzz but even with just the TV on and everything else off before the store opens I can't find a spot thats nearly as bad. I can't tell if the buzz is less because its in a room with sound deadening walls but I will be returning the one I have and picking up another in hopes they really are that different.
zakmichaels really hit the nail on the head with his description of the problem, I also used my Iphone decibel meter, probably not as accurate as the one he had but I noticed 4dB increase when it was in front of the TV. I did a test with a person slowly waving a small solid object in front of the TV and you can pin point where the buzz is emitting from and almost disappears when certain parts of the screen are covered.
On a side note I haven't seen any other 2010 buzzing Samsung's returned but did get 1 - 60PK550 LG returned due to loud buzzing. So I hope the buzzing units are few and far between as this came of the first shipment of the year and hopefully the ones we have now have a later manufacture date and newer parts, perhaps they were using left over 850 parts. I also checked the PN58C590 thats on demo and couldn't hear any buzz from it although I could hear some from the PN50C540.
For people going to see these TV's in stores make sure you aren't standing above the top of the TV when your listening for the buzz you probably won't hear it, its like flipping a light switch as your ear passes the top of the bezel when your sitting down.
I am not only disappointed in Samsung but as well as Panasonic for not offering the 5,000,000:1 - Infinite Black on any 58" TVs other than 3D models. I know they have a 54" 2D with the Infinite Black but I'm picky and want a 58" and 3D doesn't interest me.