
The first step to this hint is to configure your Mac to require a password to wake from sleep of screensaver. You can enable this setting in System Preferences in the Security control panel. Once there, choose the General options tab and check the box to 'Require password x minutes after sleep or screen saver begins.' Set the variable time to immediately require a password.
At this point, you may want to take some time to review and adjust your Screen Saver settings, and your Energy Saver settings, as this security option will also be triggered when just your display goes to sleep normally.
Quit System Preferences and you're ready for the final piece of this hint.
Next time you get up from your Mac to run to a meeting or the bathroom or whatever, fire off this keyboard combination:
Control+Shift+Eject
This is actually a shortcut to sleep your computer display, but now has the side effect of locking your screen and requiring a password to resume your session.
[crarko adds: I tested this, and it works as described. It's basically a variation of this previous hint, but has the advantage of using a default keyboard sequence. There's a number of ways that have been offered to do the screen locking, and this hint is another alternative.]