I did migration assistant from a Lion 2009 MBP to my new Lion Retina MBP, and one of the glitches was that the login screen changed to having a flat gray background color. I later upgraded to Mountain Lion, and this did not fix the problem. I tried messing around with NSTexturedFullScreenBackgroundColor.png, but that didn't help. That file appears to be the correct textured background introduced in Lion, but the screen is still just the gray. I have included an attachment showing a picture I took of the screen. Your help is appreciated.