How to get keyboard focus on login screen password field in OS X 10.8?
I have also been bothered by this and some trial and error led me to discover that the login screen keyboard focus problem is not caused by the display turning off. Setting the display to never sleep in System Preferences revealed that there is a screen saver that appears when the computer is left idle at the login screen. The login field retains focus until the screen saver appears, and when the screen saver goes away, the field no longer has focus.

This screen saver is not controlled by the screen saver settings in System Preferences, so I have not found a way to disable it. However, I did find a setting that I use as a workaround. There is a Terminal command that can be used to set the amount of idle time before the screen saver appears. I just set it to a number of seconds that is higher than the amount of time I am usually away from the computer. To change this setting you can open the Terminal (in /Applications/Utilities) and run the following command:

sudo defaults write /Library/Preferences/com.apple.screensaver loginWindowIdleTime 10800


Of course you should replace the 10800 with a number of seconds that will work for you. This command is documented by Apple Support in the article "Configuring a screen saver to run at the Login Window."

http://support.apple.com/kb/HT4625

The article refers to OS X v10.6 specifically, but I used it on Mountain Lion 10.8.