I'm no rMBP owner yet, but I have experienced the infamous scrolling lag at an Apple Store yesterday. Honestly I don't know what you all are on about.
It definitely did happen on the three 2.3/8/256/ML that I tested, but it wasn't particularly annoying as the OS remained responsive at all times. Only the area being scrolled was laggy. Meanwhile, no freeze or even the slightest peep of slow down anywhere else. Everything else kept running and moving like normal in parallel.
Out of Safari (bunch of content-heavy sites), Mail, Notes, Reminders, Finder, iTunes, a few others, gesture- and Dock-triggered animations, the only places where I noticed lags (granted, they were significant):
- Scrolling down a long, heavy Facebook page.
- Scrolling down a mail thread (right pane) in Mail, even a short single-mail one.
Worth mentioning, those two places were not perfect on a 13" MBA, meaning way better but not perfectly smooth either.
Switching spaces did stutter once or twice very briefly (noticed it because I was focusing on the smoothness, or lack thereof).
Aside from that, I hammered the poor rMBPs with gestures in plenty of apps, including long pages and relatively long and rich pages (not Facebook), expecting stutters like mentioned here, but nothing except the occasional micro-stutter while switching spaces. More importantly, stutter or not, all gestures registered instantly and animations lasted the same time (their end time wasn't offset by the stutter-induced delays).
Really, I was going to the store expecting productivity-hindering lags after having read these forums lately. In the end, the rMBPs are nothing short of mind-blowing in every way. These scrolling lags are the exception, not the rule, and even when they happen, can be looked past comfortably. Not to mention, this is all software. Given the improvements some have noticed going from Lion to Mountain Lion, one can expect further improvements in the future.