I have the same problem sometimes after screen saver activated.
I guess the problem is VirtualBox witn Win7. If last active application is VirtualBox with Win7 all keyboard stream go to in Windows but not in password field. Usually I use change user way to reactivate password field.
Also keyboard layout switcher does not work from keybord (Ctrl+Shift) complitly (I change it by mouse)
I have the same problem sometimes after screen saver activated.
I guess the problem is VirtualBox witn Win7. If last active application is VirtualBox with Win7 all keyboard stream go to in Windows but not in password field. Usually I use change user way to reactivate password field.
Also keyboard layout switcher does not work from keybord (Ctrl+Shift) complitly (I change it by mouse)