Enterprise Mac Security: Mac OS X Snow Leopard

1.6. General

Under the General tab, the first and most important of these options is the "Disable automatic login" option. Automatic login, which will remember your password and automatically log you in, is enabled by default. While this may sound incredibly convenient, it really should be disabled. Anyone with physical access to your computer would be able to restart the computer and, if the password is remembered, not be required to enter a password in order to get access to your files. With automatic login enabled, few security measures will stop someone from accessing your files if your Mac were to fall into the wrong hands.

Also under the General tab is the option to "Require a password [time interval] after sleep or screen saver begins." ...