It depends on what background is being used. I tested for IR and looked extremely close to areas I have left and was able to see IR on all the "opaque" colors. If you use the "Lake" wallpaper provided by Apple, IR will still continue to show through in most of the background. So lets say someone finds their own background and IR appears all over the place on it just like the solid dark grey? I don't think the IR has anything to do with the background. The fact is that it's there and does exist. Just because it doesn't show on your current background doesn't mean it won't show on others! I read earlier that it had something to do with the color of the dark grey, but I can prove that IR does not show on only specific colors. It will show on all colors and it just depends how easily noticeable IR becomes on certain colors.
Anyways... here is my update for the Samsung replacement I have:
Completely NO IR with Picnaut's test on the Samsung display! I'm very pleased with this screen at the moment though I do notice that whites are a bit warmer than my LG display. That doesn't bother me though because the Samsung does seem to have better color than the LG in general. I'll upload a few photos comparing the two later, but I don't have the time now. (I have to get to work! D:)
I do have a question to everyone who owns a ML equiped rMBP though. Does your screen flick to black before going to the login screen/desktop? Because both my rMBP do. This is an example of exactly how it behaves like on YouTube: http://youtu.be/931ozjd1gWU
You can see that the screen immediately turns black before entering the desktop. Is this normal because it's happening on both my LG and Samsung screens? My friend says that his Samsung display doesn't do that but mine does and I have a feeling it's software related.