I have had a couple of strange events... not really a "lock up" as much as a "stopped working". I can still use my mouse/trackpad to move the pointer around, and the keyboard is still mostly functional, but no mouse clicks/trackpad taps are effective... and on at least one occasion, I was unable to get to FILE/SAVE in the menu, which resulted in lost work when I was forced to power cycle my rMBP.
I also noticed that tapping the power button no longer invokes the screen lock function.