Didn't Tim say they had tightened up security? Also it's hard to smuggle out a 27" iMac. If its as I suspect just a speed bump and USB 3 wouldn't they just look the same anyway? Seeing a picture of a Sandy Bridge processor won't prove anything about the release date.
Nevertheless 5 days before WWDC there was a leaked list with new part numbers of the upcoming refreshes including rMBP.