The new rMBP they replace for me has none of this issue at all. Setting 50% brightness is even able to result the burn-in image on my previous rMBP.My 23" Cinema Display at work has been displaying retained images for years (probably since new and I may not have noticed the first year or more). When moving a white window aside, I can actually read the retained small text of the white window on a dark grey window behind it. The ghost image takes about 5-minutes to fade.
It bothered me a bit at first, but only psychologically. I doubt it's ever effected the quality of my photo editing or video color grading. It's never gotten better or worse and I'd also assume it's just a by-product of the technology. I suppose it would depend on just how high a contrast there is between the dark grey background and the less-dark-grey ghost.
This thread has got me curious about my HP 23" IPS display as well, which I've never bothered to analyze for ghosts. Maybe I'll check it tonight.
Anyway, I'm not going to return my rMBP (still on backorder) if it's an issue I have to go looking for, but I would if it is an issue so severe that it's constantly looking for me.
I simply demonstrated the whole progress to Genius at Apple Store yesterday. The panel kind of has some defect. I encourage you return the machine or ask for replacing once you find out any burn-in images.