Same problem here; the spacebar obviously leaves an imprint on my screen (mbpr 15"). I only noticed it a few weeks ago, when cleaning my screen. I thought it was fingerprints, but turned out to be some sort of scratch or imprint clearly in the shape of the spacebar. Apple claims it's caused by the user, but I have no idea how that is possible. I use it exclusively on a desk, where there is absolutely no way of causing excessive pressure to the screen/lid...
The computer is just over a year old, not cool...