Well, you could always make it a habit to lock your screen when you're out of the room. Checking the Keychain Access -> Preferences -> Show Status in Menu Bar checkbox puts a padlock icon among your menu extras; the first menu item is Lock Screen. Screen saver starts immediately.
That's what I'm already doing. the F keys do not trip the screensaver password box even. They just function as usual, which was a bit surprising. I can somewhat see the screen brightness being immune, but itunes play?
Yes, this is a laptop, cannot turn off or unplug keyboard.
And incidentally, I've gone one step quicker on starting my screensaver with lock. I have a dock item I can click that immediately starts the locked screensaver.