Just to add my $.02..
Bought my 15" MBP retina in July. I use it mainly for editing photos while I'm on the road or on location and randomly one day in August I noticed some ghosting. I had had my calendar open, then swiped over to my dashboard which is that dark background and noticed some IR. Didn't think much of it but I did remember it.
Then about a week after, I noticed it again when I had been viewing a photo in portrait orientation then switched to a photo that was in landscape, and I saw the old photo above and below the new one.
Then I started noticing it more and more. I think, with mine anyway, that the longer the computer is on, the warmer it gets while I'm working, and the more suseptible to burn in it is.
So I took it to Apple today after scheduling an appointment and seeing this thread. I did run the code in terminal and see that I do indeed have an LG panel. I spoke with the guy at the counter and he came back after a few minutes with an ipad in hand which had directions on it to test if a display is flawed by detecting burn-in after running a test.
He connected my laptop to an ethernet cable for their in-house internet and went to http://r140.local/display and that is an internal website that is specifically for testing image retention. It has you put the display at 90 degrees and when you hit Begin Test it displays a black and white checkerboard pattern for 3 minutes, then immediately displays a light grey background for 1 minute. If you can see the checkerboard pattern for more than 1 minute, the display is defective and they are to replace the display or the whole computer. But if it's gone within 1 minute then it's considered normal. Lame.
When we ran the test on mine, I tried telling him it'd probably not show it because my laptop was freezing cold from being in my car for a little while, then carrying it a ways through the parking lot with it being 40 degrees out. And sure enough, at the end of the 3 minutes, no pattern remained once the grey screen came up. He understood that it may be because the computer wasn't really warm yet and mentioned it in the notes for the case they've started for me, and he said that if it happens again or is still a problem to bring it back in. But if I do, I'm going to do some heavy photo editing right before so that it gets nice and toasty, because that seems to be the only time it happens.