I left the computer locked with the screensaver running, and when I came back, as soon as I moved the mouse, I had a black screen with the mouse pointer, and no prompt for password. It is impossible to unlock the screen, I have been waiting for a while to see if it goes back to the screensaver, but it does not. The only thing that I can do (also according on people on #kde) is to kill the session, losing all opened files and data (fortunately I think I didn't have any unsaved data) Checking with top, I noticed that ksmserver was taking 44% of my CPU, that seems a lot to me, I think it could be related. I'm sorry, I have no idea how to reproduce this, it has been working correctly until now (but I had kde 4.10 only for a couple of weeks). I'll add more info if I manage to reproduce this.
I killed and restarted the session without rebooting and it happened again... Since it never happened, twice in a row looks suspicious, I'll try to reboot.
I found the problem... this morning I installed and I was testing oxygen-transparent style ( https://projects.kde.org/projects/playground/artwork/oxygen-transparent ) and it looks the cause of the issue. By switching back to the oxygen style, the bug is fixed even without rebooting
I think it should be solved in oxygen-transparent git, or you can add "kscreenlocker_greet" to the blacklist" if not. *** This bug has been marked as a duplicate of bug 314227 ***