I've been following this helpful thread and am considering exchanging my cMBP 15 for the Retina 15 (a tough call given that the cMBP has no glitches). I came across some interesting info I thought worth sharing.
First, I've been looking at open box 15 rMBPs in nearby Best Buys and have been permitted to load the computer and check the screen type on two units at two different Best Buys. Both have LP-SJA2 panels (sigh, hoping for a Samsung). More interestingly, I got to talking to one of the floor guys who told me he worked for Apple for a number of years and just recently had stopped working for them (if I recall he had a couple of jobs with them, one in the tech field). I had no reason to doubt his veracity; he hadn't been the rep working with me, he wouldn't get commission (none of them do), and he wouldn't even get credit for the sale (like an hours bump) since he knew that I had primarily been working with another rep; moreover it would be an exchange so the store wouldn't make much money off it anyway.
--Anyway, he knew about the image retention issue and told me they had fixed it with the 2nd Generation (read: 2013) model. He said it was a production problem and they are now producing them all in America whereas this formerly wasn't the case. Moreover, I was talking to him about the 13 rMBP and how I had had eye trouble with it (I think it was just an adjustment thing on my part), but he mentioned that at one point they had an acknowledged display problem with the 13 -- something about how it would go in and out of focus in some way. This was in 2012 and he said Apple fixed the issue. Now obviously this is just a testimonial account, but like I said, he seemed down to earth and credible. If true, one good thing to take away is that Apple is well aware of these issues and taking steps to amend them at the production level. It may be that the LG - SJA2s can still have image retention, as some have reported here, but that the probability of it happening is much lower given production changes.--One more thing, I spoke to a rep at an Apple Store about this issue, and she knew about the IR and had actually experienced it moderately on her own 15 Retina. She also thought it was more a problem on the 1st generation (hers was a first generation).--Just some data I thought would be helpful for those of us concerned with this issue.