Everyone,
I found a video on YouTube showing the lag I think people are discussing. This is on a 2.3 GHz version of the rMBP w/ 8 GB RAM:
http://youtu.be/uixeNzzJBXQ
I believe this person was using a screen recording program, so I'm not sure how much overhead that adds and whether that could skew the results of what you are seeing on screen.
I took an actual video recording of my screen using a DSLR, to eliminate any processing overhead that a screen recording application may add. I scrolled through George Takei's Facebook timeline, which has a lot of images and comments, as well as theVerge.com which showed the choppiest scrolling in the above video.
I quickly shot off this video using the automatic settings, I wasn't going for a technically perfect video, so please forgive those shortcomings. However, I think you will see when comparing both of these, I am experiencing nothing even remotely similar to what is shown in the first video.
http://youtu.be/p5OLbKCi3ok
The only discernible differences between my RMBP and the one in the first lag video is that mine is the faster 2.6 model with double the RAM (16 GB vs 8 GB). Both machines are on 10.8 Mountain Lion using Safari 6. The other thing I did differently was that I pre-loaded the web pages to eliminate any lag associated with page or image load time. This is not something that was done in the first video, and the lag seen in that video may have something to do with the fact that the pages and images were also loading while the pages were scrolled. Pure conjecture on my part.
So as you can see, I'm not making this stuff up. I just don't see the lag that others are seeing and experiencing.