LG screen : possible Image Retention which, unlike Image Burn In, fades away in a few minutes like on most modern Plasma TVs. Other than this, the LG screens have great uniformity!
Samsung screen : Doesn't seem to be plagued with ghosting / IR issues, but has tint and light bleed issues.
Pick your poison! I'd take a LG screen over Sammy any day!
Unless you have major IR issues, just forget it and enjoy the damn computer!!!
here is a picture i took of my background...
So if you go to Apple store and purchase a rMBP, should one request Samsung/LG panel. Or it is more a russian roulette. Stuff like these leave you no choice and hope Apple make it right on the next release.