Why when I wake my new iMac from Sleep, I have to scrape away a layer of screensaver or Lion linen to log in?
I see this same problem. I see it on both user accounts on my 2011 iMac when the screen saver has been active and the user is automatically logged out. If the user logs themselves out this isn't present.

I don't believe it's a problem, just a video driver quirk.