Interesting.... I don't use gfxcardstatus and still get tearing, but this test basically confirms my suspicion.
I've had an issue with screen tearing in safari since I bought the rMBP and now that I think about it, I usually notice it after waking from sleep. Either that or if graphic switching is involved. Like if I have an app open that makes the nvidia kick in.
It only happens in Safari and it's mostly noticeable in text boxes (like posting on this forum). I notice it rarely elsewhere, like scrolling through a web page makes it look like there is a scrolling lag when it is just the related tearing.
So I think this problem is a graphic switching issue unrelated to gfxcardstatus because, again, I don't use it. I bet anybody with a rMBP could reproduce it if they don't think they have it