If you look at the box where you enter your password, there's also a button labeled "Switch User" which will bring up the LogIn window. So all you have to do is make one corner of your screen activate the screensaver. (You can make all four corners do different things just by moving the mouse pointer over each one. You do this in System Preferences under Expose and Spaces.) So when you step away from your computer, you move the mouse pointer far into a corner, the screensaver comes up and then if anyone else tries to use it, the password screen comes up. If someone doesn't have your password, they'll have to choice but to click "Switch User" which will take then to the login screen.
So you don't have to do very much, it takes no time at all to log out securely.