It happens to me not just in Safari, it even happens in the OS installation screen, when I have already erased the SSD. I had it happen to 2 units of mine, it seems not many people here on the forums have experienced it.
I have exactly the same issue and it's, for me, mostly related to Safari.
I receive my rMBP at the beginning of July with Lion installed and i did not have any flickering till i update to ML. So at this point i suspect a software issue.
It would be great if all of us having this problem tell exactly how they install their system by answering those questions:
1. first system installed
2. data and application transferred wit migration assistant (Y/N)
3. system update
In my case:
3. Mountain Lion